Add new executemany_mode, support for psycopg2.extras.execute_values()

Add new executemany_mode, support for psycopg2.extras.execute_values()

Added new dialect flag for the psycopg2 dialect, ``executemany_mode`` which
supersedes the previous experimental ``use_batch_mode`` flag.
``executemany_mode`` supports both the "execute batch" and "execute values"
functions provided by psycopg2, the latter which is used for compiled
:func:`.insert` constructs.   Pull request courtesy Yuval Dinari.

.. seealso::

    :ref:`executemany_mode`

Fixes: #4623
Closes: #4764
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4764
Pull-request-sha: c3d3a36f7eb66c86d14ed9c1c31b4b48bd204855

Change-Id: I77e26ca729f9317af1488a6c054c23fa1a6b622b