Fixes#4040
* The TCP connection is now shutdown after OnBodyFinished
* Any open connections are closed when cNetworkSingleton::Terminate() is called.
* Removed ownership cycles in cUrlClientRequest
* Added a check to the test to ensure there are no leaks.
Stop using gdb on osx - was breaking the build
Add clang 3.5 build as travis now defaults to 5.0
Fix unknown-warning-option errors on AppleClang
ProtoProxy: Use nullptr
UrlClientTest: add override to callback destructor
Update jsoncpp to use nullptr
