Feature: Support Debug Breakpoints with Akka.Cluster

[Zetanova]

I think it would be possible to enable the support of debug breakpoints in running akka cluster nodes.

We would resolve two major points for it

1. Extend the Akka.Cluster messages and the node-status to include paused nodes.
   This new node-status would not trigger dissociation and buffer flushes until deadline or resume
2. Find a possibility to execute code before and after a breakpoint gets triggered

Simply put, the node that gets debug-hold should notify the cluster that it is paused until deadline.
After the debug-resume the node informs the cluster to recover its status and transmit all buffered messages.

If point 2 is not possible to implement,
an other possibility would be to notify the cluster that a debugger attached to the node-process or detached for it.
The cluster could then react differently when a node with attached debugger gets unreachable.