eventlet: stop monkey patching

Let's stop monkey patching Python internals. It creates a lot of problem
(cf SQLAlchemy pool issues fixed by
64bc862f6fa936dbe5dd2d402240a81da40bc5a5) as nothing has ever been
designed with multi-threading in mind. It is incompatible with a lot of
library using threads internally, causing random issues like dead-locks.

Change-Id: Iad37c418be4780a7e78bf48fa080dda4ae1035b8
Revert "Ensure we are not exhausting the sqlalchemy pool"

This reverts commit 64bc862f6fa936dbe5dd2d402240a81da40bc5a5.

Now that we don't monkey patch anymore with eventlet, we're sure the
situation fixed previously by that patch where a process could consume
more messages than having SQL connection can't happen anymore.

Change-Id: Ia0f1037e1b8c3e079d382c608e047614e388ce37