Add a "counter" component to count entries by issuer

[jsha]

It's handy to know who is submitting certificates at what rate. Rather than stick this into the inclusion checker, I think we should add a new component, the "counter." This component would start out from the current STH at startup, and continually run get-entries up to the current STH.

When processing entries, this component would increment a counter with three labels:

• root: The Issuer CN of the topmost certificate in the entry's chain.
• issuer: The Issuer CN of the end-entity certificate.
• precert: True or false depending on whether the end-entity certificate is a precertificate.

[jsha]

We talked about this and concluded this isn't core to the purpose of ct-woodpecker.