Reduce log verbosity of startup/shutdown for launcher subprocesses.

There's no really good reason why the autovacuum launcher and logical
replication launcher should announce themselves at startup and shutdown
by default.  Users don't care that those processes exist, and it's
inconsistent that those background processes announce themselves while
others don't.  So, reduce those messages from LOG to DEBUG1 level.

I was sorely tempted to reduce the "starting logical replication worker
for subscription ..." message to DEBUG1 as well, but forebore for now.
Those processes might possibly be of direct interest to users, at least
until logical replication is a lot better shaken out than it is today.

Discussion: https://postgr.es/m/19479.1489121003@sss.pgh.pa.us