Remove flow_text table

Merged flow_text into flow_revision, it only exists in core for historical
reasons.  Any wiki that expects a reasonable amount of content should set
$wgFlowExternalStore to keep the bulk of content outside the revision table.

To allow storage classes to assign auto-ids the WritableObjectStorage::insert
method now returns an array representing the final storage state.
ObjectMapper::fromStorageRow has received an additional nullable param, $object,
which allows us to push those changes from insert back into the object.

Revision content compression happens in AbstractRevision::setContent,
decompression is handled in AbstractRevision::getContent so we lazily decompres
only the content the content that is needed.

Change-Id: I5cf3fe8ef0f6edb4328f4355a4abe5bec439446e
Merge "Remove flow_text table"