Add missing include in beast header
Add pending_handlers
Clean up Overlay and PeerFinder sources:
* Tidy up identifiers and declarations
* Merge PeerFinder headers into one file
* Merge handout classes and functions into one file
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.

Summary
* 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
* Tidy up Checker declaration
Load PeerFinder database in Stoppable::onPrepare:
OverlayImpl::onStart calls into PeerFinder before PeerFinder::Manager::onStart,
causing tests to sometimes fail and the application to intermittently not start.
The order of calls to Stoppable::onStart is implementation defined and not
predictable.

This changes PeerFinder to load the database in Stoppable::onPrepare, before
threads are launched. In general, creation and initialization of resources that
are shared between classes should happen in onPrepare rather than onStart,
to solve this problem.