* make sure we check all inode headers before getting an inode, so we know the latest version
* have get_mutable, put_mutable, and delete_mutable update data consistency information across all write-devices
* have get_mutable remember which driver(s) succeeded, so subsequent calls on e.g. inode resolution don't needlessly query stale data
* remember data consistency information by device ID, as well as data ID.
* get_mutable will search for the latest version of a datum across a set of known devices
* put_mutable will include the device ID in the data it replicates
* delete_mutable will delete from all devices
* datastore inodes will rely on their headers to identify the globally-latest version of the inode data
* user lists and private key indexes will track cross-device versioning so that an update from one device will always be processed by the others.
* add support for getting, setting, and deleting user state from storage providers
* add support for generating and saving per-identity and per-user private key indexes
* make sure get_mutable() operates on *non-local* data storage providers for shared data.
