Robustness

[holisticode]

This PR adds the parameter Robust to the Healthy structure.

Robust denotes

whether we are connected to a minimum of peers in all the bins we have known peers in.

It is meant as a discussion PR until we reach consensus on how this parameter will be used.

For reviewers other than @nolash :
You are welcome to leave comments but this is not yet ready for formal review.

[nolash]

For my part this is now ok. But I am not at all sure whether the idea I have of "healthiness" is the right one anymore. So I defer judgement. My advice is to have two more approvals of this besides mine from someone who understands the terms that are in use.

[zelig]

I had a discussion with @holisticode about this. IMHO healthy is primarily used as a stop condition in simulation tests
Since the current version only checks nearest neighbours, saturation of bins shallower than depth is not checked.
With another implementation of hive discovery, unhealthy kademlias could pass as healthy.
But saturation check cannot be absolute since we can have situations that there is simply less than MinBinSize peers for a node in a bin. Therefore the peer pot has to be enriched with the counts of potential peers for each bin (upto depth at least) for each node.
These cardinalities can be passed to the health function via a slice of integers which are compared to known and connected peers.

[zelig]

obsoleted by #1195