Add a means to easily check if an external signer is signing blocks as expected

[jferrant]

There are plans to add metrics to the dashboard concerning how many blocks were signed in a given reward cycle by a signer.

Create an RPC endpoint that reports the number of blocks signed by a given public key for perhaps a given reward cycle. Would involve introducing a table of signer public keys with reward cycle and number of blocks signed. As each block is processed, read the vector of signatures, and update the table accordingly.

[jcnelson]

To implement this, we would need to:

• Create a table in the chainstate DB with rows of (public key, reward cycle ID, signature count)
• Each time we process a Nakamoto block, check the signatures in its header and update the signer's (public key, reward cycle ID, signature count) row.
• Add an RPC endpoint that takes a reward cycle and public key as input, and returns the signature count

The table can be populated lazily -- i.e. the absence of a row for the given key and reward cycle can be treated the same as a row in which the signature count is zero.

[blockstack-devops]

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