Mutex on _CONFIGURE_MUTEX in automap.prepare()

Mutex on _CONFIGURE_MUTEX in automap.prepare()

Fixed a race condition which could occur if automap
:meth:`.AutomapBase.prepare` were used within a multi-threaded context
against other threads which  may call :func:`.configure_mappers` as a
result of use of other mappers.  The unfinished mapping work of automap
is particularly sensitive to being pulled in by a
:func:`.configure_mappers` step leading to errors.

Change-Id: I6d36df6639bf5cb8f137187dff68f386f5e84f88
Fixes: #4266
(cherry picked from commit 2ac7cad7179ff594d8bb3df9856c891bf4e51097)