Disconnected phones don't disappear (bugfix)

[forum thread](https://groups.google.com/forum/#!topic/serval-project-developers/y0_qQtWqle4)

`subscriber->reachable` was not updated if it was different from `subscriber_is_reachable(subscriber)`.

*The first commit is from [this pull request](https://github.com/servalproject/serval-dna/pull/64).*