Session order on segments is important in gdd tests, it determines the
cancellation order on deadlocks.  We used to create a full gang on the
BEGIN command, this was used in gdd tests to control the session orders
on segments.  However this behavior was changed during the optimization
of readonly queries, BEGIN command no longer creates a full gang.

So now we use the `RESET optimizer` command to trigger the gang
creation, we do not really care about the optimizer status although.
Reviewed-by: NZhenghua Lyu <zlv@pivotal.io>