update to latest version of Typesafe CONTRIBUTING rules
Merge pull request #1803 from akka/wip-CONTRIBUTING-∂π

update to latest version of Typesafe CONTRIBUTING rules
!per #3681 Performance and consistency improvements

- batch-write of persistent messages (user API)
- batch-write of events (in EventsourcedProcessor)
- command processing in EventsourcedProcessor by unstashing messages one-by-one from the internal stash
   * fixes performance issues that come up with unstashAll
- commands are not looped through journal actor but processed directly
- initial performance tests
  * command sourcing
  * event sourcing
  * event sourcing with user stash operations
- suppress stack traces in tests
Merge pull request #1808 from krasserm/wip-3681-performance-improvements-krasserm

!per #3681 Performance and consistency improvements