Eat XIDs more efficiently in recovery TAP test.

Eat XIDs more efficiently in recovery TAP test.

The point of this loop is to insert 1000 rows into the test table
and consume 1000 XIDs.  I can't see any good reason why it's useful
to launch 1000 psqls and 1000 backend processes to accomplish that.
Pushing the looping into a plpgsql DO block shaves about 10 seconds
off the runtime of the src/test/recovery TAP tests on my machine;
that's over 10% of the runtime of that test suite.

It is, in fact, sufficiently more efficient that we now demonstrably
need wait_slot_xmins() afterwards, or the slaves' xmins may not have
moved yet.