Allow for skipping thread_yielding

When greenthreads aren't being used this call is not
so useful and since oslo.db is a library it can't be
guaranteed that those using it will be also using
eventlet (or similar library).

Change-Id: Ib06d3761fa3f6d4d089357b396ea8c84d41ee835
Merge "Allow for skipping thread_yielding"