+ akka-cluster-metrics: new akka module

* new akka module split from akka-cluster
* provides sigar provisioning
* resolve #16121
* see #16354
well, how?
* on each class?
* on each method?
* on every scala doc?
* all of the above?

I think that would be too much noise (and needless efforts :-)).

Instead, how about if we just:
*  keep ```TODO deprecate metrics``` marker in the code, to ease location things to be removed
* say "do not use" in [documentation](http://doc.akka.io/docs/akka/snapshot/scala/cluster-usage.html)
* say "do not use" in ```reference.conf```
* set ```akka.cluster.metrics.enabled=off``` in ```reference.conf```

?