Split backend status and progress related functionality out of pgstat.c.

Backend status (supporting pg_stat_activity) and command
progress (supporting pg_stat_progress*) related code is largely
independent from the rest of pgstat.[ch] (supporting views like
pg_stat_all_tables that accumulate data over time). See also
a333476.

This commit doesn't rename the function names to make the distinction
from the rest of pgstat_ clearer - that'd be more invasive and not
clearly beneficial. If we were to decide to do such a rename at some
point, it's better done separately from moving the code as well.

Robert's review was of an earlier version.

Reviewed-By: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/20210316195440.twxmlov24rr2nxrg@alap3.anarazel.de

Author: anarazel

src/backend/bootstrap/bootstrap.c

@@ -407,8 +407,11 @@ AuxiliaryProcessMain(int argc, char *argv[])
 		 */
 		CreateAuxProcessResourceOwner();
 
-		/* Initialize backend status information */
+		/* Initialize statistics reporting */
 		pgstat_initialize();
+
+		/* Initialize backend status information */
+		pgstat_beinit();
 		pgstat_bestart();
 
 		/* register a before-shutdown callback for LWLock cleanup */