Introduce a LOG_SERVER_ONLY ereport level, which is never sent to client.

Introduce a LOG_SERVER_ONLY ereport level, which is never sent to client.

This elevel is useful for logging audit messages and similar information
that should not be passed to the client.  It's equivalent to LOG in terms
of decisions about logging priority in the postmaster log, but messages
with this elevel will never be sent to the client.

In the current implementation, it's just an alias for the longstanding
COMMERROR elevel (or more accurately, we've made COMMERROR an alias for
this).  At some point it might be interesting to allow a LOG_ONLY flag to
be attached to any elevel, but that would be considerably more complicated,
and it's not clear there's enough use-cases to justify the extra work.
For now, let's just take the easy 90% solution.

David Steele, reviewed by Fabien Coelho, Petr Jelínek, and myself