Reorganize logical replication worker disconnect code

Move the walrcv_disconnect() calls into the before_shmem_exit handler.
This makes sure the call is always made even during exit by signal, it
saves some duplicate code, and it makes the logic more similar to
walreceiver.c.

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

src/backend/replication/logical/tablesync.c

@@ -140,7 +140,6 @@ finish_sync_worker(void)
 	CommitTransactionCommand();
 
 	/* Stop gracefully */
-	walrcv_disconnect(wrconn);
 	proc_exit(0);
 }