Reduce log level of some logical decoding messages from LOG to DEBUG1

Reduce log level of some logical decoding messages from LOG to DEBUG1

Previously some logical decoding messages (e.g., "logical decoding found
consistent point") were logged at level LOG, even though they provided
low-level, developer-oriented information that DBAs were typically not
interested in.

Since these messages can occur routinely (for example, when keeping calling
pg_logical_slot_get_changes() to obtain the changes from logical decoding),
logging them at LOG can be overly verbose.

This commit reduces their log level to DEBUG1 to avoid unnecessary log noise.

This change applies to a small set of messages for now. Additional messages
may be adjusted similarly in the future.

Even with this change, if these messages from walsender still need to be
observed, enabling DEBUG1 logging selectively for walsender (e.g.,
log_min_messages = 'warning,walsender:debug1') would be helpful to avoid
increasing overall log volume.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwGTyHgtD9tyN664x6vQ8Q1G53H7ZUCgBU9_X=nLt3f1QA@mail.gmail.com