Prevent memory context logging from sending log message to connected client.

Prevent memory context logging from sending log message to connected client.

When pg_log_backend_memory_contexts() is executed, the target backend
should use LOG_SERVER_ONLY to log its memory contexts, to prevent them
from being sent to its connected client regardless of client_min_messages.
But previously the backend unexpectedly used LOG to log the message
"logging memory contexts of PID %d" and it could be sent to the client.
This is a bug in memory context logging.

To fix the bug, this commit changes that message so that it's logged with
LOG_SERVER_ONLY.

Back-patch to v14 where pg_log_backend_memory_contexts() was added.

Author: Fujii Masao
Reviewed-by: Bharath Rupireddy, Atsushi Torikoshi
Discussion: https://postgr.es/m/82c12f36-86f7-5e72-79af-7f5c37f6cad7@oss.nttdata.com