autorelay: Refactor relay_finder and start autorelay after identify

[MarcoPolo]

Fixes two flaky issues:

1. autrelay may start making connections before the identify goroutines are ready.
2. autorelay_tests are flaky because the ticker channels may be received in the wrong order.

Also uses a fork of the mock clock library to fix a data race in the library. Upstream patch coming soon.

[marten-seemann]

This looks a lot cleaner than what we had. Some suggestions to make it even cleaner.