Drop support for optional persistOnEventId in PersistOnEventRequest

[volkerstampa]

The fix for #385 introduced an optional field persistOnEventId in PersistOnEventRequest. This is optional just for backwards compatibility to existing snapshots. Once 0.9 is released applications can re-create snapshots to make sure all PersistOnEventRequests have this field defined. With an 0.10 release the implementation can be simplified by making this field mandatory (i.e. drop support for reading snapshots written with <0.9 releases).