!per #3681 Performance and consistency improvements

- batch-write of persistent messages (user API)
- batch-write of events (in EventsourcedProcessor)
- replace stash with command buffer actor for event sourcing
   * fixes denial of service (stash/unstash/receive overhead) under high load
   * throughput of event sourcing is now approx. 50% of command sourcing
- initial performance tests (disabled)
  * command sourcing
  * event sourcing
  * event sourcing with user stash operations
- suppress stack traces in tests