Compute XID horizon for page level index vacuum on primary.

Previously the xid horizon was only computed during WAL replay. That
had two major problems:
1) It relied on knowing what the table pointed to looks like. That was
   easy enough before the introducing of tableam (we knew it had to be
   heap, although some trickery around logging the heap relfilenodes
   was required). But to properly handle table AMs we need
   per-database catalog access to look up the AM handler, which
   recovery doesn't allow.
2) Not knowing the xid horizon also makes it hard to support logical
   decoding on standbys. When on a catalog table, we need to be able
   to conflict with slots that have an xid horizon that's too old. But
   computing the horizon by visiting the heap only works once
   consistency is reached, but we always need to be able to detect
   conflicts.

There's also a secondary problem, in that the current method performs
redundant work on every standby. But that's counterbalanced by
potentially computing the value when not necessary (either because
there's no standby, or because there's no connected backends).

Solve 1) and 2) by moving computation of the xid horizon to the
primary and by involving tableam in the computation of the horizon.

To address the potentially increased overhead, increase the efficiency
of the xid horizon computation for heap by sorting the tids, and
eliminating redundant buffer accesses. When prefetching is available,
additionally perform prefetching of buffers.  As this is more of a
maintenance task, rather than something routinely done in every read
only query, we add an arbitrary 10 to the effective concurrency -
thereby using IO concurrency, when not globally enabled.  That's
possibly not the perfect formula, but seems good enough for now.

Bumps WAL format, as latestRemovedXid is now part of the records, and
the heap's relfilenode isn't anymore.

Author: Andres Freund, Amit Khandekar, Robert Haas
Reviewed-By: Robert Haas
Discussion:
    https://postgr.es/m/20181212204154.nsxf3gzqv3gesl32@alap3.anarazel.de
    https://postgr.es/m/20181214014235.dal5ogljs3bmlq44@alap3.anarazel.de
    https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

src/include/access/genam.h

@@ -188,6 +188,11 @@ extern IndexScanDesc RelationGetIndexScan(Relation indexRelation,
 extern void IndexScanEnd(IndexScanDesc scan);
 extern char *BuildIndexValueDescription(Relation indexRelation,
 						   Datum *values, bool *isnull);
+extern TransactionId index_compute_xid_horizon_for_tuples(Relation irel,
+									 Relation hrel,
+									 Buffer ibuf,
+									 OffsetNumber *itemnos,
+									 int nitems);
 
 /*
  * heap-or-index access to system catalogs (in genam.c)

src/include/access/hash_xlog.h

@@ -263,7 +263,7 @@ typedef struct xl_hash_init_bitmap_page
  */
 typedef struct xl_hash_vacuum_one_page
 {
-	RelFileNode hnode;
+	TransactionId latestRemovedXid;
 	int			ntuples;
 
 	/* TARGET OFFSET NUMBERS FOLLOW AT THE END */

src/include/access/heapam.h

@@ -174,6 +174,10 @@ extern void simple_heap_update(Relation relation, ItemPointer otid,
 
 extern void heap_sync(Relation relation);
 
+extern TransactionId heap_compute_xid_horizon_for_tuples(Relation rel,
+									ItemPointerData *items,
+									int nitems);
+
 /* in heap/pruneheap.c */
 extern void heap_page_prune_opt(Relation relation, Buffer buffer);
 extern int heap_page_prune(Relation relation, Buffer buffer,

src/include/access/nbtxlog.h

@@ -126,8 +126,7 @@ typedef struct xl_btree_split
  */
 typedef struct xl_btree_delete
 {
-	RelFileNode hnode;			/* RelFileNode of the heap the index currently
-								 * points at */
+	TransactionId latestRemovedXid;
 	int			nitems;
 
 	/* TARGET OFFSET NUMBERS FOLLOW AT THE END */

src/include/access/tableam.h

@@ -299,6 +299,12 @@ typedef struct TableAmRoutine
 											 TupleTableSlot *slot,
 											 Snapshot snapshot);
 
+	/* see table_compute_xid_horizon_for_tuples() */
+	TransactionId (*compute_xid_horizon_for_tuples) (Relation rel,
+													 ItemPointerData *items,
+													 int nitems);
+
+
 	/* ------------------------------------------------------------------------
 	 * Manipulations of physical tuples.
 	 * ------------------------------------------------------------------------
@@ -689,6 +695,19 @@ table_tuple_satisfies_snapshot(Relation rel, TupleTableSlot *slot, Snapshot snap
 	return rel->rd_tableam->tuple_satisfies_snapshot(rel, slot, snapshot);
 }
 
+/*
+ * Compute the newest xid among the tuples pointed to by items. This is used
+ * to compute what snapshots to conflict with when replaying WAL records for
+ * page-level index vacuums.
+ */
+static inline TransactionId
+table_compute_xid_horizon_for_tuples(Relation rel,
+									 ItemPointerData *items,
+									 int nitems)
+{
+	return rel->rd_tableam->compute_xid_horizon_for_tuples(rel, items, nitems);
+}
+
 
 /* ----------------------------------------------------------------------------
  *  Functions for manipulations of physical tuples.

src/include/access/xlog_internal.h

@@ -31,7 +31,7 @@
 /*
  * Each page of XLOG file has a header like this:
  */
-#define XLOG_PAGE_MAGIC 0xD099	/* can be used as WAL version indicator */
+#define XLOG_PAGE_MAGIC 0xD100	/* can be used as WAL version indicator */
 
 typedef struct XLogPageHeaderData
 {