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