MDEV-4506: Parallel replication. Intermediate commit.

Hook in the wait-for-prior-commit logic (not really tested yet).
Clean up some resource maintenance around rpl_group_info (may still be some
smaller issues there though).
Add a ToDo list at the top of rpl_parallel.cc

sql/sql_class.h

@@ -1659,6 +1659,7 @@ struct wait_for_commit
   void unregister_wait_for_prior_commit2();
 
   wait_for_commit();
+  ~wait_for_commit();
 };