pgsql: Disable STARTUP_PROGRESS_TIMEOUT in standby mode.

Disable STARTUP_PROGRESS_TIMEOUT in standby mode.

In standby mode, we don't actually report progress of recovery,
but up until now, startup_progress_timeout_handler() nevertheless
got called every log_startup_progress_interval seconds. That's
an unnecessary expense, so avoid it.

Report by Thomas Munro. Patch by Bharath Rupireddy, reviewed by
Simon Riggs, Thomas Munro, and me. Back-patch to v15, where
the problem was introduced.

Discussion: https://www.postgresql.org/message-id/CA%2BhUKGKCHSffAj8zZJKJvNX7ygnQFxVD6wm1d-2j3fVw%2BMafPQ%40mail.gmail.com

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/98e7234242a652497c99d4d0d6f2bf9a75d4e921

Modified Files
--------------
src/backend/access/transam/xlogrecovery.c | 25 ++++++++++++++++++++++---
src/backend/postmaster/startup.c | 30 +++++++++++++++++++++++++++---
src/include/postmaster/startup.h | 2 ++
3 files changed, 51 insertions(+), 6 deletions(-)