Report progress of CREATE INDEX operations

This uses the progress reporting infrastructure added by c16dc1aca5,
adding support for CREATE INDEX and CREATE INDEX CONCURRENTLY.

There are two pieces to this: one is index-AM-agnostic, and the other is
AM-specific.  The latter is fairly elaborate for btrees, including
reportage for parallel index builds and the separate phases that btree
index creation uses; other index AMs, which are much simpler in their
building procedures, have simplistic reporting only, but that seems
sufficient, at least for non-concurrent builds.

The index-AM-agnostic part is fairly complete, providing insight into
the CONCURRENTLY wait phases as well as block-based progress during the
index validation table scan.  (The index validation index scan requires
patching each AM, which has not been included here.)

Reviewers: Rahila Syed, Pavan Deolasee, Tatsuro Yamada
Discussion: https://postgr.es/m/20181220220022.mg63bhk26zdpvmcj@alvherre.pgsql

src/backend/access/nbtree/nbtutils.c

@@ -20,6 +20,7 @@
 #include "access/nbtree.h"
 #include "access/reloptions.h"
 #include "access/relscan.h"
+#include "commands/progress.h"
 #include "miscadmin.h"
 #include "utils/array.h"
 #include "utils/datum.h"
@@ -2051,6 +2052,29 @@ btproperty(Oid index_oid, int attno,
 	}
 }
 
+/*
+ *	btbuildphasename() -- Return name of index build phase.
+ */
+char *
+btbuildphasename(int64 phasenum)
+{
+	switch (phasenum)
+	{
+		case PROGRESS_CREATEIDX_SUBPHASE_INITIALIZE:
+			return "initializing";
+		case PROGRESS_BTREE_PHASE_INDEXBUILD_TABLESCAN:
+			return "scanning table";
+		case PROGRESS_BTREE_PHASE_PERFORMSORT_1:
+			return "sorting live tuples";
+		case PROGRESS_BTREE_PHASE_PERFORMSORT_2:
+			return "sorting dead tuples";
+		case PROGRESS_BTREE_PHASE_LEAF_LOAD:
+			return "loading tuples in tree";
+		default:
+			return NULL;
+	}
+}
+
 /*
  *	_bt_truncate() -- create tuple without unneeded suffix attributes.
  *