Reduce the default value of autovacuum_vacuum_cost_delay to 2ms.

This is a better way to implement the desired change of increasing
autovacuum's default resource consumption.

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

doc/src/sgml/config.sgml

@@ -7033,7 +7033,7 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
         Specifies the cost delay value that will be used in automatic
         <command>VACUUM</command> operations.  If -1 is specified, the regular
         <xref linkend="guc-vacuum-cost-delay"/> value will be used.
-        The default value is 20 milliseconds.
+        The default value is 2 milliseconds.
         This parameter can only be set in the <filename>postgresql.conf</filename>
         file or on the server command line;
         but the setting can be overridden for individual tables by

src/backend/utils/misc/guc.c

@@ -3312,7 +3312,7 @@ static struct config_real ConfigureNamesReal[] =
 			GUC_UNIT_MS
 		},
 		&autovacuum_vac_cost_delay,
-		20, -1, 100,
+		2, -1, 100,
 		NULL, NULL, NULL
 	},
 

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

@@ -589,7 +589,7 @@
 #autovacuum_multixact_freeze_max_age = 400000000	# maximum multixact age
 					# before forced vacuum
 					# (change requires restart)
-#autovacuum_vacuum_cost_delay = 20ms	# default vacuum cost delay for
+#autovacuum_vacuum_cost_delay = 2ms	# default vacuum cost delay for
 					# autovacuum, in milliseconds;
 					# -1 means use vacuum_cost_delay
 #autovacuum_vacuum_cost_limit = -1	# default vacuum cost limit for