Introduce logical decoding.

This feature, building on previous commits, allows the write-ahead log
stream to be decoded into a series of logical changes; that is,
inserts, updates, and deletes and the transactions which contain them.
It is capable of handling decoding even across changes to the schema
of the effected tables.  The output format is controlled by a
so-called "output plugin"; an example is included.  To make use of
this in a real replication system, the output plugin will need to be
modified to produce output in the format appropriate to that system,
and to perform filtering.

Currently, information can be extracted from the logical decoding
system only via SQL; future commits will add the ability to stream
changes via walsender.

Andres Freund, with review and other contributions from many other
people, including Álvaro Herrera, Abhijit Menon-Sen, Peter Gheogegan,
Kevin Grittner, Robert Haas, Heikki Linnakangas, Fujii Masao, Abhijit
Menon-Sen, Michael Paquier, Simon Riggs, Craig Ringer, and Steve
Singer.

src/include/access/rewriteheap.h

@@ -14,12 +14,14 @@
 #define REWRITE_HEAP_H
 
 #include "access/htup.h"
+#include "storage/itemptr.h"
+#include "storage/relfilenode.h"
 #include "utils/relcache.h"
 
 /* struct definition is private to rewriteheap.c */
 typedef struct RewriteStateData *RewriteState;
 
-extern RewriteState begin_heap_rewrite(Relation NewHeap,
+extern RewriteState begin_heap_rewrite(Relation OldHeap, Relation NewHeap,
 				   TransactionId OldestXmin, TransactionId FreezeXid,
 				   MultiXactId MultiXactCutoff, bool use_wal);
 extern void end_heap_rewrite(RewriteState state);
@@ -27,4 +29,29 @@ extern void rewrite_heap_tuple(RewriteState state, HeapTuple oldTuple,
 				   HeapTuple newTuple);
 extern bool rewrite_heap_dead_tuple(RewriteState state, HeapTuple oldTuple);
 
+/*
+ * On-Disk data format for an individual logical rewrite mapping.
+ */
+typedef struct LogicalRewriteMappingData
+{
+	RelFileNode		old_node;
+	RelFileNode		new_node;
+	ItemPointerData	old_tid;
+	ItemPointerData	new_tid;
+} LogicalRewriteMappingData;
+
+/* ---
+ * The filename consists out of the following, dash separated,
+ * components:
+ * 1) database oid or InvalidOid for shared relations
+ * 2) the oid of the relation
+ * 3) xid we are mapping for
+ * 4) upper 32bit of the LSN at which a rewrite started
+ * 5) lower 32bit of the LSN at which a rewrite started
+ * 6) xid of the xact performing the mapping
+ * ---
+ */
+#define LOGICAL_REWRITE_FORMAT "map-%x-%x-%X_%X-%x-%x"
+void CheckPointLogicalRewriteHeap(void);
+
 #endif   /* REWRITE_HEAP_H */