Make event views bookkeeping thread-safe
... and some minor cleanup and optimization of the ViewTester and the EventSlot.
@bwynne, FYI. Please review. I would recommend to simplify the bookkeeping even further, at your earliest convenience, to minimize the contention during sub-slot registration. The web of containers you added is probably a bit too bulky for this design.
This closes #36 (closed), which is one of the possible manifestations of the actual problem being mitigated by this MR.