Refactor PeerFinder:

Previously, the PeerFinder manager constructed with a Callback object provided by the owner which was used to perform operations like connecting, disconnecting, and sending messages. This made it difficult to change the overlay code because a single call into the PeerFinder could cause both OverlayImpl and PeerImp to be re-entered one or more times, sometimes while holding a recursive mutex.  This change eliminates the callback by changing PeerFinder functions to return values indicating the action the caller should take.

As a result of this change the PeerFinder no longer needs its own dedicated thread. OverlayImpl is changed to call into PeerFinder on a timer to perform periodic activities. Furthermore the Checker class used to perform connectivity checks has been refactored. It no longer uses an abstract base class, in order to not type-erase the handler passed to async_connect (ensuring compatibility with coroutines). To allow unit tests that don't need a network, the Logic class is now templated on the Checker type. Currently the Manager provides its own io_service. However, this can easily be changed so that the io_service is provided upon construction.

Changes:
* Remove unused SiteFiles dependency injection
* Remove Callback and update signatures for public APIs
* Remove obsolete functions
* Move timer to overlay
* Steps toward a shared io_service
* Templated, simplified Checker

Reviewers: @ximinez @howardhinnant

Note: The two oldest commits are from #614