Add bulk_replace to AssociationSet, AssociationDict

Add bulk_replace to AssociationSet, AssociationDict

Implemented a more comprehensive assignment operation (e.g. "bulk replace")
when using association proxy with sets or dictionaries.  Fixes the problem
of redundant proxy objects being created to replace the old ones, which
leads to excessive events and SQL and in the case of unique constraints
will cause the flush to fail.

Fixes: #2642
Change-Id: I57ab27dd9feba057e539267722cce92254fca777