Avoid redundant reads of head in peakNode

Motivation:

There is not need todo redunant reads of head in peakNode as we can just spin on next() until it becomes visible.

Modifications:

Remove redundant reads of head in peakNode. This is based on @nitsanw's patch for akka.
See https://github.com/akka/akka/pull/15596

Result:

Less volatile access.