Introduce a single log_audit_event(AuditEvent *) function

Now the various event sources (triggers, hooks) create AuditEvents and
pass them to log_audit_event(), which decides whether to log them, and
how. This is better than sprinkling ereport()s everywhere, and it makes
it possible to describe the log format concisely as well.

Also gets rid of the make_timestamp hack along with minor cleanups.
Describe the log format in the README