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/include/pgstat.h

@@ -951,10 +951,11 @@ typedef enum ProgressCommandType
 {
 	PROGRESS_COMMAND_INVALID,
 	PROGRESS_COMMAND_VACUUM,
-	PROGRESS_COMMAND_CLUSTER
+	PROGRESS_COMMAND_CLUSTER,
+	PROGRESS_COMMAND_CREATE_INDEX
 } ProgressCommandType;
 
-#define PGSTAT_NUM_PROGRESS_PARAM	10
+#define PGSTAT_NUM_PROGRESS_PARAM	20
 
 /* ----------
  * Shared-memory data structures