cmd/addrwitness: a new addressable or bastion-mediated witness

[FiloSottile]

This uses internal/witness like cmd/witness, but with a few major differences:

• in general, addrwitness is designed to be directly addressable from the Internet, without a fetcher, enabling low-latency logs to fetch co-signatures synchronously and distribute them along with their tree heads
• optionally, addrwitness supports being exposed through a bastion, for witnesses that are not easily exposed to the Internet
• the witness adds two signatures: a plain one for backwards compatibility, and a timestamped one
• the get API returns the tree size of the latest checkpoint, not the whole checkpoint, to prevent misuse and direct logs to take care of co-signature distribution
• logs are indexed by the origin string, and there is no concept of a separate log ID
  • this is important and arguably a security fix: see the comments in main.go or this

This PR is a proof of concept. In particular, it doesn't support compact ranges or ECDSA signatures, and has no tests or metrics, and its handling of reconnection to the bastion is clunky.

[AlCutter]

It would be nice if this could be made to fit inside a feeder implementation as it would then Just Work™️ inside the omniwitness etc.

e.g. the sumdb feeder - the fetchCheckpoint and fetchProof funcs could poll the bastion (e.g. getCheckpoint polls for CP+poof, returns CP and stores the proof to be returned by the next call to fetchProof or something?)

Might need a bit of refactoring or some sprinkles on the configuration front...