This PR introduces clique_status which gives info about the health of
the clique network.

It's currently a bit PITA to find out how a clique network is
performing, and it can easily happen that sealers drop off -- and
everything is 'fine' until one more signer drops off, and the network
suddenly halts.

The new method provides the following stats:

- Which signers are currently active, and have signed blocks in the last
  N (set to 64) blocks?
- How many blocks has each signer signed?
- What is the difficulty in the last N blocks, compared to the
  theoretical maximum?