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/storage/ipc/standby.c

@@ -401,7 +401,7 @@ ResolveRecoveryConflictWithLock(LOCKTAG locktag)
 		 */
 		VirtualTransactionId *backends;
 
-		backends = GetLockConflicts(&locktag, AccessExclusiveLock);
+		backends = GetLockConflicts(&locktag, AccessExclusiveLock, NULL);
 		ResolveRecoveryConflictWithVirtualXIDs(backends,
 											   PROCSIG_RECOVERY_CONFLICT_LOCK);
 	}