sources: allow selection of unreachable sources

sources: allow selection of unreachable sources

Reachability is no longer a requirement for selection. An unreachable
source can remain selected if its newest sample is not older than the
oldest sample from all reachable sources.

This is useful to prevent reselection when an accurate source uses a
very short polling interval (e.g. refclock) and is occasionally
unreachable for short periods of time.