Keep a hashset of queued/processed event ids

[Charles Solar]

In a situation such as this:

1. EventStore delivers event to endpoint
2. Endpoint loses connection to store
3. Endpoint processes event and queues up ACK
4. Endpoint reconnects to store

there could be a race condition here:

5. Endpoint ACKs processed event
5. EventStore re-delivers event to endpoint

[Charles Solar]

Fixed in v0.17 - agg.net no longer delays the ACK and queues events