Add a new benchmark measuring the append and purge performance of the
MemoryEventStore. This benchmark revealed that the store is orders of
magnitude slower than it should be due to conservative validation
(hugely so: 300KB/s vs 568MB/s). Turn off this validation by default.
For #190
This CL utilizes the event store to write outgoing messages and removes
the unbounded outgoing data structure.
It also adds a new interface [EventStore.Open]
For #190
Introduce an EventStore interface to store events for resumable streams.
Provide an in-memory implementation.
Still to do: connect to streaming transports.
For #10
Export the Event type in preparation for providing user-definable
storage for resumable streams.
Also, move event code to a separate file.
This PR is just renaming and code motion.
