Introduce validator jailing

[shawn-zil]

The main job of a validator is to propose and vote. If a validator is bad e.g. it has not proposed/voted for a while - jail it.

When a validator is jailed, it will be skipped when selecting the next leader. The validator is released from jail when it starts behaving good again e.g. it starts to vote/propose.

[DrZoltanFazekas]

@shawn-zil The whitepaper outlines the idea of jailing (liveness penalty by suspending a validator's right to propose blocks) in section VIII.C but we will re-design the algorithm before implementing it.