Change to the Application io_service:
* Simplified the implementation and removed class IoServicePool
* The io_service outlives the components of the Application
Fix Overlay stop on exit:
The stop sequence for Overlay had a race condition where autoconnect could
be called after close_all, resulting in a hang on exit. This resolves the
problem by putting the close and timer operations on a strand:
* Rename some Overlay members
* Put close on strand and tidy up members
* Use completion handler instead of coroutine for timer
* Use App io_service in PeerFinder