Rollback mitigation may produce obsolete data events

When rollback mitigation is enabled, if the connection drops while there are unpersisted events in the stream event buffer, the stream will be reopened starting from the last persisted event. This is by design. However, the event buffer is not cleared, so it contains obsolete entries from the previous stream.

In the worst case, this could result in events from a rolled-back alternate history being propagated to the data event handler.

Whenever a new stream is opened, we should make sure the stream event buffer for that vbucket does not contain obsolete events.