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

Builds/VisualStudio2013/RippleD.vcxproj

@@ -2837,13 +2837,8 @@
     </ClCompile>
     <ClInclude Include="..\..\src\ripple\peerfinder\impl\Bootcache.h">
     </ClInclude>
-    <ClCompile Include="..\..\src\ripple\peerfinder\impl\Checker.cpp">
-      <ExcludedFromBuild>True</ExcludedFromBuild>
-    </ClCompile>
     <ClInclude Include="..\..\src\ripple\peerfinder\impl\Checker.h">
     </ClInclude>
-    <ClInclude Include="..\..\src\ripple\peerfinder\impl\CheckerAdapter.h">
-    </ClInclude>
     <ClCompile Include="..\..\src\ripple\peerfinder\impl\Config.cpp">
       <ExcludedFromBuild>True</ExcludedFromBuild>
     </ClCompile>

Builds/VisualStudio2013/RippleD.vcxproj.filters

@@ -3984,15 +3984,9 @@
     <ClInclude Include="..\..\src\ripple\peerfinder\impl\Bootcache.h">
       <Filter>ripple\peerfinder\impl</Filter>
     </ClInclude>
-    <ClCompile Include="..\..\src\ripple\peerfinder\impl\Checker.cpp">
-      <Filter>ripple\peerfinder\impl</Filter>
-    </ClCompile>
     <ClInclude Include="..\..\src\ripple\peerfinder\impl\Checker.h">
       <Filter>ripple\peerfinder\impl</Filter>
     </ClInclude>
-    <ClInclude Include="..\..\src\ripple\peerfinder\impl\CheckerAdapter.h">
-      <Filter>ripple\peerfinder\impl</Filter>
-    </ClInclude>
     <ClCompile Include="..\..\src\ripple\peerfinder\impl\Config.cpp">
       <Filter>ripple\peerfinder\impl</Filter>
     </ClCompile>