Update AbstractNodeQueue.java

Avoid spinning on head when a peek/poll observes a null next but the queue is not empty.
Update AbstractNodeQueue.java

language language....
+act #15502: Pluggable DNS resolution in akka-io
+act #13004 Adding TailChopping router
=con #15600 Fix config in PeekMailboxSpec
!scr do not remove tags (github) that are not set on an issue

Since the updated github issues views, it shows all "removed tags",
events in the issue even if the removed tag was never set on the issue.
So this script would always show up as "removed validating,needs-attention"
Even if the issue never had the label needs-attention set to it,
which is confusing visually.

This change introduces that we only remove tags that actually are set on
the issue.
Merge pull request #15369 from kjanosz/tail-chopping-router

+act #13004 Adding TailChopping router
Merge pull request #15602 from ktoso/github-tagging-impr-ktoso

!scr do not remove tags (github) that are not set on an issue
Merge pull request #15417 from hajile/master

+act #15502: Pluggable DNS resolution in akka-io
Merge pull request #15601 from LeonardMeyer/LMeyer-specfix-#15600

=con #15600 Fix config in PeekMailboxSpec
Add abbrev for project changes to CONTRIBUTING.md

As noticed during https://github.com/akka/akka/pull/15617, we didn't hint people to use `=pro` when changing things in the project / build.
=pro #15616 UTF-8 charset and exception with offending file (for Validation)
Merge pull request #15623 from ktoso/port-15616-master-ktoso

=pro #15616 UTF-8 charset and exception with offending file (for Validation)
=doc #15565 improved wording in cluster-usage docs
Merge pull request #15624 from ktoso/docs-cluster-joining-ktoso

=doc #15565 improved wording in cluster-usage docs
=con #15613 Rename one of the DistributedPubSubMediatorSpec classes
=doc Fix typo in scala remoting.rst
Merge pull request #15630 from savulchik/patch-1

Fix typo in scala remoting.rst
+con #13737 Interceptors chain for Actors' bahavior.

* Add new pattern to akka-contrib.
* Mixin ReceivePipeline trait into Actors that have to be intercepted.
Merge pull request #15629 from akka/wip-15613-rename-DistributedPubSubMediatorSpec-patriknw

=con #15613 Rename one of the DistributedPubSubMediatorSpec classes