Implement peer ejection

[mikera]

Need to complete work to eject peers in following circumstances:

• Unable to keep up with network load
• Network dropout
• Insufficient stake to have continued presence

Ejection probably works best with an active step taken by another participant, e.g. (eject-peer 0xa2.....). This takes the computational cost of continuously evaluating peers off-chain.

There should be some (probably quite small) penalty for being ejected (maybe 0.1% of stake) as well as a small incentive for other parties to execute the ejection (e.g. getting the memory refund).