pg_prewarm: Add automatic prewarm feature.

Periodically while the server is running, and at shutdown, write out a
list of blocks in shared buffers.  When the server reaches consistency
-- unfortunatey, we can't do it before that point without breaking
things -- reload those blocks into any still-unused shared buffers.

Mithun Cy and Robert Haas, reviewed and tested by Beena Emerson,
Amit Kapila, Jim Nasby, and Rafia Sabih.

Discussion: http://postgr.es/m/CAD__OugubOs1Vy7kgF6xTjmEqTR4CrGAv8w+ZbaY_+MZeitukw@mail.gmail.com

src/backend/storage/buffer/freelist.c

@@ -168,6 +168,23 @@ ClockSweepTick(void)
 	return victim;
 }
 
+/*
+ * have_free_buffer -- a lockless check to see if there is a free buffer in
+ *					   buffer pool.
+ *
+ * If the result is true that will become stale once free buffers are moved out
+ * by other operations, so the caller who strictly want to use a free buffer
+ * should not call this.
+ */
+bool
+have_free_buffer()
+{
+	if (StrategyControl->firstFreeBuffer >= 0)
+		return true;
+	else
+		return false;
+}
+
 /*
  * StrategyGetBuffer
  *