Use replay position as floor for WAIT FOR LSN standby_(write|flush)

Use replay position as floor for WAIT FOR LSN standby_(write|flush)

GetCurrentLSNForWaitType() for standby_write and standby_flush modes
returned only the walreceiver position, which may lag behind WAL
already present on the standby from a base backup, archive restore,
or prior streaming.  This could cause unnecessary blocking if the
target LSN falls between the walreceiver's tracked position and the
replay position.

Fix by returning the maximum of the walreceiver position and the
replay position.  WAL up to the replay point is physically on disk
regardless of its origin, so there is no reason to wait for the
walreceiver to re-receive it.

This complements 29e7dbf5e4d, which seeded writtenUpto to
receiveStart in RequestXLogStreaming() to fix the most common
hang scenario.  The getter-level floor handles the remaining edge
cases: targets between receiveStart and the replay position, and
standbys running with archive recovery only (no walreceiver).

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