Add bgw_type field to background worker structure.  It is intended to be
set to the same value for all workers of the same type, so they can be
grouped in pg_stat_activity, for example.

The backend_type column in pg_stat_activity now shows bgw_type for a
background worker.  The ps listing also no longer calls out that a
process is a background worker but just show the bgw_type.  That way,
being a background worker is more of an implementation detail now that
is not shown to the user.  However, most log messages still refer to
'background worker "%s"'; otherwise constructing sensible and
translatable log messages would become tricky.
Reviewed-by: Michael Paquier <michael.paquier@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>