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/test/regress/expected/rules.out

@@ -1747,6 +1747,28 @@ pg_stat_database_conflicts| SELECT d.oid AS datid,
     pg_stat_get_db_conflict_bufferpin(d.oid) AS confl_bufferpin,
     pg_stat_get_db_conflict_startup_deadlock(d.oid) AS confl_deadlock
    FROM pg_database d;
+pg_stat_progress_vacuum| SELECT s.pid,
+    s.datid,
+    d.datname,
+    s.relid,
+        CASE s.param1
+            WHEN 0 THEN 'initializing'::text
+            WHEN 1 THEN 'scanning heap'::text
+            WHEN 2 THEN 'vacuuming indexes'::text
+            WHEN 3 THEN 'vacuuming heap'::text
+            WHEN 4 THEN 'cleaning up indexes'::text
+            WHEN 5 THEN 'truncating heap'::text
+            WHEN 6 THEN 'performing final cleanup'::text
+            ELSE NULL::text
+        END AS phase,
+    s.param2 AS heap_blks_total,
+    s.param3 AS heap_blks_scanned,
+    s.param4 AS heap_blks_vacuumed,
+    s.param5 AS index_vacuum_count,
+    s.param6 AS max_dead_tuples,
+    s.param7 AS num_dead_tuples
+   FROM (pg_stat_get_progress_info('VACUUM'::text) s(pid, datid, relid, param1, param2, param3, param4, param5, param6, param7, param8, param9, param10)
+     JOIN pg_database d ON ((s.datid = d.oid)));
 pg_stat_replication| SELECT s.pid,
     s.usesysid,
     u.rolname AS usename,