fix delete mutation returning incorrect data (fix #1794) (fix #1763) (#1827)

From `alpha-40` we've been using a `WHERE` clause to fetch required rows and generate mutation response. This has a few limitations like the requirement of a primary key/unique constraint. This also returns inconsistent data on `delete` mutation as mentioned in #1794. 
Now, we're using `VALUES (..)` (refer [here](https://www.postgresql.org/docs/current/sql-values.html)) expression to form virtual table rows in `SQL` to generate mutation response.

Internal changes:-
- Not to use primary key/unique constraint columns:-
  - Revert back to `ConstraintName` from `TableConstraint` in `TableInfo` type
  - Remove `tcCols` field in `TableConstraint` type
  - Modify `table_info.sql` and `fetchTableMeta` function `SQL`
- A test case to perform `delete` mutation and returning relational objects.

docs/graphql/manual/api-reference/mutation.rst

@@ -238,34 +238,11 @@ Mutation Response
 ^^^^^^^^^^^^^^^^^
 .. code-block:: none
 
-    # if table has atleast one primary key or
-    # one unique constraint with not null columns
     {
       affected_rows
       returning {
-        col-field1
-        col-field2
-        ..
-        relation1{
-          relation1-field1
-          relation1-field2
-          ..
-        }
-        relation2{
-          relation2-field1
-          relation2-field2
-          ..
-        }
-        ..
-      }
-    }
-
-    # if table has no primary key or unique constraints
-    {
-      affected_rows
-      returning {
-        col-field1
-        col-field2
+        response-field1
+        response-field2
         ..
       }
     }
@@ -274,22 +251,6 @@ E.g.:
 
 .. code-block:: graphql
 
-    # if table has atleast one primary key or
-    # one unique constraint with not null columns
-    {
-      affected_rows
-      returning {
-        id
-        author_id
-        articles{
-          id
-          title
-          content
-        }
-      }
-    }
-
-    # if table has no primary key or unique constraints
     {
       affected_rows
       returning {
@@ -305,8 +266,6 @@ E.g.:
 
 .. code-block:: none
 
-    # if table has atleast one primary key or
-    # one unique constraint with not null columns
     objects: [
       {
         field1: value,
@@ -323,23 +282,12 @@ E.g.:
       },
       ..
     ]
-
-    # if table has no primary key or unique constraints
-    objects: [
-      {
-         col_field1: value,
-         col_field2: value
-         ..
-      },
-      ..
-    ]
+    # no nested objects
 
 E.g.:
 
 .. code-block:: graphql
     
-    # if table has atleast one primary key or
-    # one unique constraint with not null columns
     objects: [
       {
         title: "Software is eating the world",
@@ -353,14 +301,6 @@ E.g.:
       }
     ]
 
-    # if table has no primary key or unique constraints
-    objects: [
-      {
-        title: "Software is eating the world",
-        content: "This week, Hewlett-Packard..."
-      }
-    ]
-
 .. _ConflictClause:
 
 **on_conflict** argument

docs/graphql/manual/mutations/delete.rst

@@ -35,10 +35,8 @@ See the :ref:`delete mutation API reference <delete_syntax>` for the full specif
 
 .. note::
 
-  - If a table is not in the ``public`` Postgres schema, the delete mutation field will be of the format
-    ``delete_<schema_name>_<table_name>``.
-  - To fetch nested objects using relationships in the mutation response, the table needs to have either a primary
-    key or a unique constraint with not null columns.
+  If a table is not in the ``public`` Postgres schema, the delete mutation field will be of the format
+  ``delete_<schema_name>_<table_name>``.
 
 Delete based on an object's fields
 ----------------------------------

docs/graphql/manual/mutations/insert.rst

@@ -36,10 +36,8 @@ See the :ref:`insert mutation API reference <insert_upsert_syntax>` for the full
 
 .. note::
 
-  - If a table is not in the ``public`` Postgres schema, the insert mutation field will be of the format
-    ``insert_<schema_name>_<table_name>``.
-  - To fetch nested objects using relationships in the mutation response, the table needs to have either a primary
-    key or a unique constraint with not null columns.
+  If a table is not in the ``public`` Postgres schema, the insert mutation field will be of the format
+  ``insert_<schema_name>_<table_name>``.
 
 Insert a single object
 ----------------------
@@ -217,10 +215,6 @@ Insert an object and get a nested object in response
       }
     }
 
-.. note::
-  
-  For this to work, the parent table (*in this case,* ``article``) needs to have either a primary key or a
-  unique constraint.
 
 Insert an object and its nested object in the same mutation
 -----------------------------------------------------------
@@ -276,11 +270,6 @@ in the response
       }
     }
 
-.. note::
-  
-  For this to work, the parent table (*in this case,* ``article``) needs to have either a primary key or a
-  unique constraint.
-
 Insert an object with a JSONB column
 ------------------------------------
 **Example:** Insert a new ``author`` object with a JSONB ``address`` column

docs/graphql/manual/mutations/update.rst

@@ -39,10 +39,9 @@ See the :ref:`update mutation API reference <update_syntax>` for the full specif
 
   - At least any one of ``_set``, ``_inc`` operators or the jsonb operators ``_append``, ``_prepend``, ``_delete_key``,
     ``_delete_elem``, ``_delete_at_path`` is required.
+
   - If a table is not in the ``public`` Postgres schema, the update mutation field will be of the format
     ``update_<schema_name>_<table_name>``.
-  - To fetch nested objects using relationships in the mutation response, the table needs to have either a primary
-    key or a unique constraint with not null columns.
 
 Update based on an object's fields
 ----------------------------------

docs/graphql/manual/mutations/upsert.rst

@@ -145,10 +145,6 @@ You can specify ``on_conflict`` clause while inserting nested objects
       }
     }
 
-.. note::
-  
-  For this to work, the parent table (*in this case,* ``author``) needs to have either a primary key or a
-  unique constraint.
 
 .. admonition:: Edge-cases