Code review comment for lp:~raof/mir/make-default-surface-store-threadsafe

The motivation isn't quite as clear as commit message implies: the SessionMediator instances are all acting on the IPC thread. But it does make sense to make this threadsafe.

However, if we're going to introduce a smart RAII based lock (instead of sprinkling a few lock_guards in the access methods) I'd prefer the code using it to look nicer than:

    return (**(store.lock()))[id];

If you can make that, for example: "return (*store)[id]" I'll go along.