Improve WAIT FOR LSN test coverage

Improve WAIT FOR LSN test coverage

Add regression coverage for several WAIT FOR LSN edge cases.

First, cover fresh walreceiver shared-memory initialization after a
standby restart.  Restart the standby while its upstream is down, so
RequestXLogStreaming() seeds writtenUpto/flushedUpto to the
segment-aligned receiveStart and the walreceiver cannot immediately
advance them.  Verify that the seeded flush position is segment-aligned,
that replay can be ahead of it, and that standby_write/standby_flush
still succeed for an already-replayed LSN via the replay-position floor
in GetCurrentLSNForWaitType().

Second, add fencepost checks for the target <= currentLSN predicate.
With replay paused and walreceiver stopped, verify exact boundaries for
standby_replay using pg_last_wal_replay_lsn(), and for standby_flush
using pg_last_wal_receive_lsn().  Also verify that a waiter for
current + 1 sleeps while replay is paused and wakes with success once
new WAL is delivered and replay advances.

Finally, add a cascading-standby timeline-switch test.  Start a waiter
on the downstream standby, promote its upstream, generate WAL on the new
timeline, and verify that the cascade follows the new timeline and the
wait completes successfully once replay reaches the target LSN.

Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/1957514.1775526774%40sss.pgh.pa.us
Author: Alexander Korotkov <aekorotkov@gmail.com>
Author: Xuneng Zhou <xunengzhou@gmail.com>