pgsql: Fix walsender to exit promptly if client requests shutdown.

Fix walsender to exit promptly if client requests shutdown.

It's possible for WalSndWaitForWal to be asked to wait for WAL that doesn't
exist yet. That's fine, in fact it's the normal situation if we're caught
up; but when the client requests shutdown we should not keep waiting.
The previous coding could wait indefinitely if the source server was idle.

In passing, improve the rather weak comments in this area, and slightly
rearrange some related code for better readability.

Back-patch to 9.4 where this code was introduced.

Discussion: https://postgr.es/m/14154.1498781234@sss.pgh.pa.us

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e9d4aa594f2caa8c28d55c41c9926420b1efdb79

Modified Files
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src/backend/replication/walsender.c | 48 ++++++++++++++++++++++++-------------
1 file changed, 31 insertions(+), 17 deletions(-)