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?
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