Allow log_min_messages to be set per process type
 
Change log_min_messages from being a single element to a comma-separated
list of type:level elements, with 'type' representing a process type,
and 'level' being a log level to use for that type of process.  The list
must also have a freestanding level specification which is used for
process types not listed, which convenientely makes the whole thing
backwards-compatible.
 
Some choices made here could be contested; for instance, we use the
process type `backend` to affect regular backends as well as dead-end
backends and the standalone backend, and `autovacuum` means both the
launcher and the workers.  I think it's largely sensible though, and it
can easily be tweaked if desired.
 
Author: Euler Taveira <euler@eulerto.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Japin Li <japinli@hotmail.com>
Reviewed-by: Tan Yang <332696245@qq.com>
Discussion: https://postgr.es/m/e85c6671-1600-4112-8887-f97a8a5d07b2@app.fastmail.com