Improve logging of autovacuum I/O activity

This adds some I/O stats to the logging of autovacuum (when the
operation takes long enough that log_autovacuum_min_duration causes it
to be logged), so that it is easier to tune.  Notably, it adds buffer
I/O counts (hits, misses, dirtied) and read and write rate.

Authors: Greg Smith and Noah Misch

src/backend/utils/init/globals.c

@@ -115,6 +115,10 @@ int			VacuumCostPageDirty = 20;
 int			VacuumCostLimit = 200;
 int			VacuumCostDelay = 0;
 
+int			VacuumPageHit = 0;
+int			VacuumPageMiss = 0;
+int			VacuumPageDirty = 0;
+
 int			VacuumCostBalance = 0;		/* working state for vacuum */
 bool		VacuumCostActive = false;