Allow cluster_name in log_line_prefix

Hi folks

I was recently surprised to notice that log_line_prefix doesn't support a
cluster_name placeholder. I suggest adding one. If I don't hear objections
I'll send a patch.

Before anyone asks "but why?!":

* A constant (short) string in log_line_prefix is immensely useful when
working with logs from multi-node systems. Whether that's physical
streaming replication, logical replication, Citus, whatever, it doesn't
matter. It's worth paying the small storage price for sanity when looking
at logs.

* Yes you can embed it directly into log_line_prefix. But then it gets
copied by pg_basebackup or whatever you're using to clone standbys etc, so
you can easily land up with multiple instances reporting the same name.
This rather defeats the purpose.

--
Craig Ringer http://www.2ndQuadrant.com/
2ndQuadrant - PostgreSQL Solutions for the Enterprise