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/commands/vacuum.c

@@ -214,6 +214,9 @@ vacuum(VacuumStmt *vacstmt, Oid relid, bool do_toast,
 
 		VacuumCostActive = (VacuumCostDelay > 0);
 		VacuumCostBalance = 0;
+		VacuumPageHit = 0;
+		VacuumPageMiss = 0;
+		VacuumPageDirty = 0;
 
 		/*
 		 * Loop to process each selected relation.