[ReactNative] Refactor BatchedBridge and MessageQueue

Summary:
@public

The current implementation of `MessageQueue` is huge, over-complicated and spread
across `MethodQueue`, `MethodQueueMixin`, `BatchedBridge` and `BatchedBridgeFactory`

Refactored in a simpler way, were it's just a `MessageQueue` class and `BatchedBridge`
is only an instance of it.

Test Plan:
I had to make some updates to the tests, but no real update to the native side.
There's also tests covering the `remoteAsync` methods, and more integration tests for UIExplorer.
Verified whats being used by Android, and it should be safe, also tests Android tests have been pretty reliable.

Manually testing: Create a big hierarchy, like `<ListView>` example. Use the `TimerMixin` example to generate multiple calls.
Test the failure callback on the `Geolocation` example.

All the calls go through this entry point, so it's hard to miss if it's broken.

React/Base/RCTBridge.m

@@ -40,9 +40,6 @@ typedef NS_ENUM(NSUInteger, RCTBridgeFields) {
   RCTBridgeFieldRequestModuleIDs = 0,
   RCTBridgeFieldMethodIDs,
   RCTBridgeFieldParamss,
-  RCTBridgeFieldResponseCBIDs,
-  RCTBridgeFieldResponseReturnValues,
-  RCTBridgeFieldFlushDateMillis
 };
 
 typedef NS_ENUM(NSUInteger, RCTJavaScriptFunctionKind) {
@@ -1258,14 +1255,6 @@ RCT_INNER_BRIDGE_ONLY(_invokeAndProcessModule:(__unused NSString *)module
     return;
   }
 
-  NSUInteger bufferRowCount = [requestsArray count];
-  NSUInteger expectedFieldsCount = RCTBridgeFieldResponseReturnValues + 1;
-
-  if (bufferRowCount != expectedFieldsCount) {
-    RCTLogError(@"Must pass all fields to buffer - expected %zd, saw %zd", expectedFieldsCount, bufferRowCount);
-    return;
-  }
-
   for (NSUInteger fieldIndex = RCTBridgeFieldRequestModuleIDs; fieldIndex <= RCTBridgeFieldParamss; fieldIndex++) {
     id field = [requestsArray objectAtIndex:fieldIndex];
     if (![field isKindOfClass:[NSArray class]]) {