Mutex the declarative scan/map process against configure_mappers()

Mutex the declarative scan/map process against configure_mappers()

Applied the mapper "configure mutex" against the declarative class mapping
process, to guard against the race which can occur if mappers are used
while dynamic module import schemes are still in the process of configuring
mappers for related classes.  This does not guard against all possible race
conditions, such as if the concurrent import has not yet encountered the
dependent classes as of yet, however it guards against as much as possible
within the SQLAlchemy declarative process.

Fixes: #4686
Change-Id: I0349036b8078bd42265ab40862cfbfe5bf9d5b44