Support PrefetchBuffer() in recovery.

Provide PrefetchSharedBuffer(), a variant that takes SMgrRelation, for
use in recovery.  Rename LocalPrefetchBuffer() to PrefetchLocalBuffer()
for consistency.

Add a return value to all of these.  In recovery, tolerate and report
missing files, so we can handle relations unlinked before crash recovery
began.  Also report cache hits and misses, so that callers can do faster
buffer lookups and better I/O accounting.

Reviewed-by: Alvaro Herrera <alvherre@2ndquadrant.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKGJ4VJN8ttxScUFM8dOKX0BrBiboo5uz1cq%3DAovOddfHpA%40mail.gmail.com

src/backend/storage/buffer/localbuf.c

@@ -54,17 +54,17 @@ static Block GetLocalBufferStorage(void);
 
 
 /*
- * LocalPrefetchBuffer -
+ * PrefetchLocalBuffer -
  *	  initiate asynchronous read of a block of a relation
  *
  * Do PrefetchBuffer's work for temporary relations.
  * No-op if prefetching isn't compiled in.
  */
-void
-LocalPrefetchBuffer(SMgrRelation smgr, ForkNumber forkNum,
+PrefetchBufferResult
+PrefetchLocalBuffer(SMgrRelation smgr, ForkNumber forkNum,
 					BlockNumber blockNum)
 {
-#ifdef USE_PREFETCH
+	PrefetchBufferResult result = {InvalidBuffer, false};
 	BufferTag	newTag;			/* identity of requested block */
 	LocalBufferLookupEnt *hresult;
 
@@ -81,12 +81,18 @@ LocalPrefetchBuffer(SMgrRelation smgr, ForkNumber forkNum,
 	if (hresult)
 	{
 		/* Yes, so nothing to do */
-		return;
+		result.recent_buffer = -hresult->id - 1;
+	}
+	else
+	{
+#ifdef USE_PREFETCH
+		/* Not in buffers, so initiate prefetch */
+		smgrprefetch(smgr, forkNum, blockNum);
+		result.initiated_io = true;
+#endif							/* USE_PREFETCH */
 	}
 
-	/* Not in buffers, so initiate prefetch */
-	smgrprefetch(smgr, forkNum, blockNum);
-#endif							/* USE_PREFETCH */
+	return result;
 }