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.
