Support parallel bitmap heap scans.

The index is scanned by a single process, but then all cooperating
processes can iterate jointly over the resulting set of heap blocks.
In the future, we might also want to support using a parallel bitmap
index scan to set up for a parallel bitmap heap scan, but that's a
job for another day.

Dilip Kumar, with some corrections and cosmetic changes by me.  The
larger patch set of which this is a part has been reviewed and tested
by (at least) Andres Freund, Amit Khandekar, Tushar Ahuja, Rafia
Sabih, Haribabu Kommi, Thomas Munro, and me.

Discussion: http://postgr.es/m/CAFiTN-uc4=0WxRGfCzs-xfkMYcSEWUC-Fon6thkJGjkh9i=13A@mail.gmail.com

src/backend/executor/execParallel.c

@@ -25,6 +25,7 @@
 
 #include "executor/execParallel.h"
 #include "executor/executor.h"
+#include "executor/nodeBitmapHeapscan.h"
 #include "executor/nodeCustom.h"
 #include "executor/nodeForeignscan.h"
 #include "executor/nodeSeqscan.h"
@@ -217,6 +218,10 @@ ExecParallelEstimate(PlanState *planstate, ExecParallelEstimateContext *e)
 				ExecCustomScanEstimate((CustomScanState *) planstate,
 									   e->pcxt);
 				break;
+			case T_BitmapHeapScanState:
+				ExecBitmapHeapEstimate((BitmapHeapScanState *) planstate,
+									   e->pcxt);
+				break;
 			default:
 				break;
 		}
@@ -277,6 +282,11 @@ ExecParallelInitializeDSM(PlanState *planstate,
 				ExecCustomScanInitializeDSM((CustomScanState *) planstate,
 											d->pcxt);
 				break;
+			case T_BitmapHeapScanState:
+				ExecBitmapHeapInitializeDSM((BitmapHeapScanState *) planstate,
+											d->pcxt);
+				break;
+
 			default:
 				break;
 		}
@@ -775,6 +785,10 @@ ExecParallelInitializeWorker(PlanState *planstate, shm_toc *toc)
 				ExecCustomScanInitializeWorker((CustomScanState *) planstate,
 											   toc);
 				break;
+			case T_BitmapHeapScanState:
+				ExecBitmapHeapInitializeWorker(
+									 (BitmapHeapScanState *) planstate, toc);
+				break;
 			default:
 				break;
 		}