pgsql: Minimal fix for WAIT FOR ... MODE 'standby_flush'

Minimal fix for WAIT FOR ... MODE 'standby_flush'

The investigation into the negative test performance impact of 7e8aeb9e483
lead to discovering that there are a few issues with WAIT FOR.

This commit is just a minimal fix to prevent hangs in standby_flush mode, due
to WAIT FOR ... 'standby_flush' seeing a 0 LSN if a newly started walreceiver
does not receive any writes, because the stanby is already caught up.

There are several other issues and this is isn't necessarily the best fix. But
this way we get the hangs out of the way.

Reported-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/zqbppucpmkeqecfy4s5kscnru4tbk6khp3ozqz6ad2zijz354k@w4bdf4z3wqoz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/29e7dbf5e4daa8fafc2b18a1551e7b31c8847340

Modified Files
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src/backend/replication/walreceiver.c | 2 --
src/backend/replication/walreceiverfuncs.c | 1 +
2 files changed, 1 insertion(+), 2 deletions(-)