Adjust log level of logical decoding messages by context

Adjust log level of logical decoding messages by context

Commit 21b018e7eab lowered some logical decoding messages from LOG to DEBUG1.
However, per discussion on pgsql-hackers, messages from background activity
(e.g., walsender or slotsync worker) should remain at LOG, as they are less
frequent and more likely to indicate issues that DBAs should notice.

For foreground SQL functions (e.g., pg_logical_slot_peek_binary_changes()),
keep these messages at DEBUG1 to avoid excessive log noise. They can still be
enabled by lowering client_min_messages or log_min_messages for the session.

This commit updates logical decoding to log these messages at LOG for
background activity and at DEBUG1 for foreground execution.

Suggested-by: Robert Haas <robertmhaas@gmail.com>
Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/CA+TgmoYsu2+YAo9eLGkDp5VP-pfQ-jOoX382vS4THKHeRTNgew@mail.gmail.com