Revert "Increase the default vacuum_cost_limit from 200 to 2000"

This reverts commit bd09503e63.

Per discussion, it seems like what we should do instead is to
reduce the default value of autovacuum_vacuum_cost_delay by the
same factor.  That's functionally equivalent as long as the
platform can accurately service the smaller delay request, which
should be true on anything released in the last 10 years or more.
And smaller, more-closely-spaced delays are better in terms of
providing a steady I/O load.

Discussion: https://postgr.es/m/28720.1552101086@sss.pgh.pa.us

doc/src/sgml/config.sgml

@@ -1932,7 +1932,7 @@ include_dir 'conf.d'
        <listitem>
         <para>
          The accumulated cost that will cause the vacuuming process to sleep.
-         The default value is 2000.
+         The default value is 200.
         </para>
        </listitem>
       </varlistentry>

src/backend/utils/init/globals.c

@@ -137,7 +137,7 @@ int			MaxBackends = 0;
 int			VacuumCostPageHit = 1;	/* GUC parameters for vacuum */
 int			VacuumCostPageMiss = 10;
 int			VacuumCostPageDirty = 20;
-int			VacuumCostLimit = 2000;
+int			VacuumCostLimit = 200;
 double		VacuumCostDelay = 0;
 
 int			VacuumPageHit = 0;

src/backend/utils/misc/guc.c

@@ -2265,7 +2265,7 @@ static struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&VacuumCostLimit,
-		2000, 1, 10000,
+		200, 1, 10000,
 		NULL, NULL, NULL
 	},
 

src/backend/utils/misc/postgresql.conf.sample

@@ -159,7 +159,7 @@
 #vacuum_cost_page_hit = 1		# 0-10000 credits
 #vacuum_cost_page_miss = 10		# 0-10000 credits
 #vacuum_cost_page_dirty = 20		# 0-10000 credits
-#vacuum_cost_limit = 2000		# 1-10000 credits
+#vacuum_cost_limit = 200		# 1-10000 credits
 
 # - Background Writer -