Route bulk update/delete exec through new Query._execute_crud method

Route bulk update/delete exec through new Query._execute_crud method

Added support for bulk :meth:`.Query.update` and :meth:`.Query.delete`
to the :class:`.ShardedQuery` class within the horiziontal sharding
extension.  This also adds an additional expansion hook to the
bulk update/delete methods :meth:`.Query._execute_crud`.

Fixes: #4196
Change-Id: I65f56458176497a8cbdd368f41b879881f06348b