Add before_mapper_configured event

Add before_mapper_configured event

This event is intended to allow a specific mapper to be skipped during the
configure step, by returning a value of `.orm.interfaces.EXT_SKIP` which means
the mapper will be skipped within this configure run. The "new mappers" flag
will remain set in this case and the configure operation will occur again.

This event, and its return value, make it possible to query one base while a
different one still needs configuration, which cannot be completed at this
time.

Fixes: #4397
Change-Id: I122e556f6a4ff842ad15315dcf39e19bb7f9a744
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4403