Convert elog(LOG) calls to ereport() where appropriate

Convert elog(LOG) calls to ereport() where appropriate

User-visible log messages should go through ereport(), so they are
subject to translation.  Many remaining elog(LOG) calls are really
debugging calls.

Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://www.postgresql.org/message-id/flat/92d6f545-5102-65d8-3c87-489f71ea0a37%40enterprisedb.com