Add a note that rpc_workers option is experimental

blueprint multiple-rpc-workers

Closes-Bug: 1304374
Change-Id: Id354b96fc591752e1e79b072997423b6447d5de9
Correctly inherit __table_args__ from parent class

Where unique constraints are added to certain tables by populating
__table_args__, we need to ensure we inherit the top level
__table_args__ that define the default storage engine to use when
creating tables.

Change-Id: Ib886b84922830b60282b4f882591e43f5041e713
Fixes: bug 1288358
(cherry picked from commit 72e37ddcd067432675543c8f4d327fe766bc8044)
Merge "Add a note that rpc_workers option is experimental" into milestone-proposed
Merge "Correctly inherit __table_args__ from parent class" into milestone-proposed