Replace package `github.com/benbjohnson/clock` with `github.com/filecoin-project/go-clock`

[gammazero]

The package github.com/benbjohnson/clock is archived, i.e. no longer maintained, and should be replaced with a fork that is maintained. The github.com/filecoin-project/go-clock is such a fork and has fixes to existing issues in the original package.