Add support for temporary replication slots

This allows creating temporary replication slots that are removed
automatically at the end of the session or on error.

From: Petr Jelinek <petr.jelinek@2ndquadrant.com>

src/include/nodes/replnodes.h

@@ -55,6 +55,7 @@ typedef struct CreateReplicationSlotCmd
 	char	   *slotname;
 	ReplicationKind kind;
 	char	   *plugin;
+	bool		temporary;
 	bool		reserve_wal;
 } CreateReplicationSlotCmd;