Add simple VACUUM progress reporting.

There's a lot more that could be done here yet - in particular, this
reports only very coarse-grained information about the index vacuuming
phase - but even as it stands, the new pg_stat_progress_vacuum can
tell you quite a bit about what a long-running vacuum is actually
doing.

Amit Langote and Robert Haas, based on earlier work by Vinayak Pokale
and Rahila Syed.

src/include/commands/progress.h

@@ -0,0 +1,37 @@
+/*-------------------------------------------------------------------------
+ *
+ * progress.h
+ *	  Constants used with the progress reporting facilities defined in
+ *	  pgstat.h.  These are possibly interesting to extensions, so we
+ *	  expose them via this header file.  Note that if you update these
+ *	  constants, you probably also need to update the views based on them
+ *	  in system_views.sql.
+ *
+ * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/commands/progress.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PROGRESS_H
+#define PROGRESS_H
+
+/* Progress parameters for (lazy) vacuum */
+#define PROGRESS_VACUUM_PHASE					0
+#define PROGRESS_VACUUM_TOTAL_HEAP_BLKS			1
+#define PROGRESS_VACUUM_HEAP_BLKS_SCANNED		2
+#define PROGRESS_VACUUM_HEAP_BLKS_VACUUMED		3
+#define PROGRESS_VACUUM_NUM_INDEX_VACUUMS		4
+#define PROGRESS_VACUUM_MAX_DEAD_TUPLES			5
+#define PROGRESS_VACUUM_NUM_DEAD_TUPLES			6
+
+/* Phases of vacuum (as advertised via PROGRESS_VACUUM_PHASE) */
+#define PROGRESS_VACUUM_PHASE_SCAN_HEAP			1
+#define PROGRESS_VACUUM_PHASE_VACUUM_INDEX		2
+#define PROGRESS_VACUUM_PHASE_VACUUM_HEAP		3
+#define PROGRESS_VACUUM_PHASE_INDEX_CLEANUP		4
+#define PROGRESS_VACUUM_PHASE_TRUNCATE			5
+#define PROGRESS_VACUUM_PHASE_FINAL_CLEANUP		6
+
+#endif