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/gist/gist.c

@@ -86,6 +86,7 @@ gisthandler(PG_FUNCTION_ARGS)
 	amroutine->amcostestimate = gistcostestimate;
 	amroutine->amoptions = gistoptions;
 	amroutine->amproperty = gistproperty;
+	amroutine->ambuildphasename = NULL;
 	amroutine->amvalidate = gistvalidate;
 	amroutine->ambeginscan = gistbeginscan;
 	amroutine->amrescan = gistrescan;

src/backend/access/gist/gistbuild.c

@@ -205,7 +205,7 @@ gistbuild(Relation heap, Relation index, IndexInfo *indexInfo)
 	/*
 	 * Do the heap scan.
 	 */
-	reltuples = table_index_build_scan(heap, index, indexInfo, true,
+	reltuples = table_index_build_scan(heap, index, indexInfo, true, true,
 									   gistBuildCallback,
 									   (void *) &buildstate, NULL);