- clean up the batch insert thing

- clean up the batch insert thing
- add a test for batch inserts
- don't need elaborate _inserted_primary_key thing
- take some cruft out of ExecutionContext, ResultProxy,
EC members can be non-underscored, have mapper just call the
EC members for now.
- simplify "connection_callable", no need for a "flush_opts"
dictionary since this point of expansion is not needed