049_wait_for_lsn.pl: create function and procedure at once

049_wait_for_lsn.pl: create function and procedure at once

Create the PL/pgSQL function and procedure for the top-level WAIT FOR
checks in a single transaction, then wait once for standby replay before
running both tests.  Also revise some surrounding comments.

This avoids an extra 'wait_for_catchup()' on the delayed standby without
changing the test coverage.

Discussion: https://postgr.es/m/CABPTF7WZ1yuYz8V%3Dxsbghg8e7qaAm5MpyNw6BthWcbN7%2BP6biw%40mail.gmail.com
Author: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>