Make coarsetime dependency and traffic-timestamping non-optional.

Previously coarsetime and the traffic-timestamp feature were enabled, since they were only required for a small corner of the guardmgr algorithm.

But in 1.0 and beyond we'll be adding a bunch of other features (eg, netflow padding, DoS prevention) that will need coarsetime all over the place.

And since we're going to be doing coarsetime all over the place, the previous justification for making traffic-timestamping optional (the tiny performance hit) is no longer relevant.