Allow VACUUM to be run with index cleanup disabled.

This commit adds a new reloption, vacuum_index_cleanup, which
controls whether index cleanup is performed for a particular
relation by default.  It also adds a new option to the VACUUM
command, INDEX_CLEANUP, which can be used to override the
reloption.  If neither the reloption nor the VACUUM option is
used, the default is true, as before.

Masahiko Sawada, reviewed and tested by Nathan Bossart, Alvaro
Herrera, Kyotaro Horiguchi, Darafei Praliaskouski, and me.
The wording of the documentation is mostly due to me.

Discussion: http://postgr.es/m/CAD21AoAt5R3DNUZSjOoXDUY=naYPUOuffVsRzuTYMz29yLzQCA@mail.gmail.com

src/include/commands/vacuum.h

@@ -148,6 +148,19 @@ typedef enum VacuumOption
 	VACOPT_DISABLE_PAGE_SKIPPING = 1 << 7	/* don't skip any pages */
 } VacuumOption;
 
+/*
+ * A ternary value used by vacuum parameters.
+ *
+ * DEFAULT value is used to determine the value based on other
+ * configurations, e.g. reloptions.
+ */
+typedef enum VacOptTernaryValue
+{
+	VACOPT_TERNARY_DEFAULT = 0,
+	VACOPT_TERNARY_DISABLED,
+	VACOPT_TERNARY_ENABLED,
+} VacOptTernaryValue;
+
 /*
  * Parameters customizing behavior of VACUUM and ANALYZE.
  *
@@ -167,6 +180,8 @@ typedef struct VacuumParams
 	int			log_min_duration;	/* minimum execution threshold in ms at
 									 * which  verbose logs are activated, -1
 									 * to use default */
+	VacOptTernaryValue index_cleanup;	/* Do index vacuum and cleanup,
+										* default value depends on reloptions */
 } VacuumParams;
 
 /* GUC parameters */

src/include/utils/rel.h

@@ -266,6 +266,7 @@ typedef struct StdRdOptions
 	AutoVacOpts autovacuum;		/* autovacuum-related options */
 	bool		user_catalog_table; /* use as an additional catalog relation */
 	int			parallel_workers;	/* max number of parallel workers */
+	bool		vacuum_index_cleanup;	/* enables index vacuuming and cleanup */
 } StdRdOptions;
 
 #define HEAP_MIN_FILLFACTOR			10