Update "clock"

[Stebalien]

We use a raul's fork of the mock clock library. Unfortunately, raul's fork is out of date and upstream is unmaintained. We either need to update the fork, or switch to usptream.

IMO, we should just switch to upstream. It "works" and doesn't appear to need any real changes. That would match libp2p (which uses upstream now).

[rvagg]

I looked in to this last year for lassie and while the current clock library is OK, it's not perfect, but https://github.com/itbasis/go-clock might be a viable alternative. It's the fork with the most attention so far and seems to be maintained. I know libp2p uses this a bit, maybe they have an alternative. @sukunrt do you have an alternative approach yet?