V2 RpcApi should register when db pool is enabled

The rpc api controller registers the public methods of db_api
But, use_tpool is True, the thread pool wrapper's methods are
used which would lead to failures.

Fixes Bug #1221037

Change-Id: Iaba10178c7da1548257e2180658c2dc8ba3e048b
Merge "V2 RpcApi should register when db pool is enabled"