Add fast execution helper support.

Add fast execution helper support.

Added a new flag ``use_batch_mode`` to the psycopg2 dialect.  This flag
enables the use of psycopg2's ``psycopg2.extras.execute_batch``
extension when the :class:`.Engine` calls upon ``cursor.executemany()``.
This extension provides a critical performance increase by over an order of magnitude
when running INSERT statements in batch.  The flag is False by default
as it is considered to be experimental for now.

Change-Id: Ib88d28bc792958d47109f644ff1d08c897db4ff7
Fixes: #4109