Implement timestamp-aware merge strategy (HIP-3)

[andrewgdotcom]

When upserting, we should determine which version of the key has the oldest self-signature and remove all third-party signatures on that key before merging. This prevents third-party signatures from growing without limit.

https://github.com/hockeypuck/hockeypuck/wiki/HIP-3:-Timestamp-aware-merge-strategy

[andrewgdotcom]

We should only do this if the keyserver no-modify flag is set on the newest self-signature. This will still effectively make it the default behaviour, but will allow advanced users to turn it off if they need to.

Q: should this be supported per-uid, and how do we deal with direct third-party sigs?

[andrewgdotcom]

Beware that in a race condition scenario, the two sides of the domain wall (wherever it falls) will fail to recon fully with each other. This will be fine so long as the blocklist delta plus the race condition delta at each mutual don't exceed the seenCache.