reorg bulk persistence into a separate module

reorg bulk persistence into a separate module

This restores persistence.py to only functions that are used
by unitofwork.py, and all the "bulk" stuff gets its own
module bulk_persistence.py.  Also fixes up the ORM context
class hierarchy for bulk.

This is all ahead of the ORM-insert changes coming in, so that
the later review can be about logic and not about reorganization.

Change-Id: I035896e9e77fcece866d246edf30097cccad0182