When ON, ORCA will optimize DML queries by enforcing a non-master gather
whenever possible. When off, a gather on master will be enforced
instead.

Default value will be ON.

Also add new tests to ensure sane behavior when this optimization is
turned on and fix the existing tests.
Signed-off-by: NSambitesh Dash <sdash@pivotal.io>
Signed-off-by: NDhanashree Kashid <dkashid@pivotal.io>