Add new replication mode synchronous_commit = 'write'.

Replication occurs only to memory on standby, not to disk,
so provides additional performance if user wishes to
reduce durability level slightly. Adds concept of multiple
independent sync rep queues.

Fujii Masao and Simon Riggs

src/include/access/xact.h

@@ -55,6 +55,7 @@ typedef enum
 {
 	SYNCHRONOUS_COMMIT_OFF,		/* asynchronous commit */
 	SYNCHRONOUS_COMMIT_LOCAL_FLUSH,		/* wait for local flush only */
+	SYNCHRONOUS_COMMIT_REMOTE_WRITE,		/* wait for local flush and remote write */
 	SYNCHRONOUS_COMMIT_REMOTE_FLUSH		/* wait for local and remote flush */
 }	SyncCommitLevel;