- organize persistence methods in terms of generators,
narrow down argument lists and generator items for each function
down to just what each function needs.   This will help for them
to be of more multipurpose use for bulk operations
- further reorganize collect_insert_commands to distinguish between
setting up given values vs. defaults.  again trying to shoot for
making this of more general use
- move out checks for table in mapper._pks_by_table
- optimize collection of cols we insert as none