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/backend/replication/repl_scanner.l

@@ -98,6 +98,7 @@ PHYSICAL			{ return K_PHYSICAL; }
 RESERVE_WAL			{ return K_RESERVE_WAL; }
 LOGICAL				{ return K_LOGICAL; }
 SLOT				{ return K_SLOT; }
+TEMPORARY			{ return K_TEMPORARY; }
 
 ","				{ return ','; }
 ";"				{ return ';'; }