Remove eventlet tpool from common db.api

There is only one place in oslo.db where we use eventlet - wrapping of
DB API method calls to be executed by tpool threads.
It makes sense to remove this feature from oslo.db code and
use the tpool.Proxy() wrapper in the db.api module in each project.

blueprint oslo-db-lib

POC in Nova - Id7de85dce11f5ea61dfccddf80de7800a2bc4e37

Change-Id: I6fa140f2b1f03c43ae7cab98ac8a12d71188af76