selectinload omit join

selectinload omit join

The "selectin" loader strategy now omits the JOIN in the case of a
simple one-to-many load, where it instead relies upon the foreign key
columns of the related table in order to match up to primary keys in
the parent table.   This optimization can be disabled by setting
the :paramref:`.relationship.omit_join` flag to False.
Many thanks to Jayson Reis for the efforts on this.

As part of this change, horizontal shard no longer relies upon
the _mapper_zero() method to get the query-bound mapper, instead
using the more generalized _bind_mapper() (which will use mapper_zero
if no explicit FROM is present).  A short check for the particular
recursive condition is added to BundleEntity and it no longer assigns
itself as the "namespace" to its ColumnEntity objects which creates
a reference cycle.

Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Fixes: #4340
Change-Id: I649587e1c07b684ecd63f7d10054cd165891baf4
Pull-request: https://bitbucket.org/zzzeek/sqlalchemy/pull-requests/7