Implement NOWAIT option for BASE_BACKUP command

Specifying this option makes the server not wait for the
xlog to be archived, or emit a warning that it can't,
instead leaving the responsibility with the client.

This is useful when the log is being streamed using
the streaming protocol in parallel with the backup,
without having log archiving enabled.

src/backend/replication/repl_scanner.l

@@ -60,6 +60,7 @@ BASE_BACKUP			{ return K_BASE_BACKUP; }
 FAST			{ return K_FAST; }
 IDENTIFY_SYSTEM		{ return K_IDENTIFY_SYSTEM; }
 LABEL			{ return K_LABEL; }
+NOWAIT			{ return K_NOWAIT; }
 PROGRESS			{ return K_PROGRESS; }
 WAL			{ return K_WAL; }
 START_REPLICATION	{ return K_START_REPLICATION; }