Add clearing of last processed message from tail node to avoid memory leaks...

...when queue is empty and no new messages come.

It don't affect throughput and latency noticeably, but adds 'var' definition to sources...

Thank you, @viktorklang ! 
https://github.com/akka/akka/commit/fb2decbcda5dd2b18a2abfbc0425d18e1e780f24#L0R40