Break out peertracker to its own library/repo under github.com/spiffe

[kfox1111]

As new agents using the Delegated Identity API come about, they need to have a way to track peers. The existing code for peer tracking in the SPIRE code base is useful for this purpose. It doesn't change much, so it shouldn't be very burdensome to break it out to its own library. Is this something we could do?