Buffer events between publisher and subscriber

[slashdotdash]

Prevent a subscriber to a single or all streams from being overloaded with published events. A limited size buffer should be used by the subscription.

Each subscriber will ack received events (#18), so the subscription knows how far behind it is. This can be used as a form of back pressure.

A subscription can catch up by going to storage after a spike in published events that it cannot keep up with.

The subscription will buffer events until the subscriber is ready to receive, or an overflow occurs. At which point it will move into a catch-up mode and query events and replay them from storage until caught up.