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/include/storage/bufmgr.h

@@ -46,6 +46,15 @@ typedef enum
 								 * replay; otherwise same as RBM_NORMAL */
 } ReadBufferMode;
 
+/*
+ * Type returned by PrefetchBuffer().
+ */
+typedef struct PrefetchBufferResult
+{
+	Buffer		recent_buffer;	/* If valid, a hit (recheck needed!) */
+	bool		initiated_io;	/* If true, a miss resulting in async I/O */
+} PrefetchBufferResult;
+
 /* forward declared, to avoid having to expose buf_internals.h here */
 struct WritebackContext;
 
@@ -162,8 +171,11 @@ extern PGDLLIMPORT int32 *LocalRefCount;
 /*
  * prototypes for functions in bufmgr.c
  */
-extern void PrefetchBuffer(Relation reln, ForkNumber forkNum,
-						   BlockNumber blockNum);
+extern PrefetchBufferResult PrefetchSharedBuffer(struct SMgrRelationData *smgr_reln,
+												 ForkNumber forkNum,
+												 BlockNumber blockNum);
+extern PrefetchBufferResult PrefetchBuffer(Relation reln, ForkNumber forkNum,
+										   BlockNumber blockNum);
 extern Buffer ReadBuffer(Relation reln, BlockNumber blockNum);
 extern Buffer ReadBufferExtended(Relation reln, ForkNumber forkNum,
 								 BlockNumber blockNum, ReadBufferMode mode,