Fix possible double free in ThreadPool

This makes HTTPRequestTest::GETThreadPool test pass deterministicaly.

Problem was static slot used to cleanup threads. When ThreadPool was instantiated
multiple times, threads were also clened up multiple times resulting in segfault.

Switch from boost shared pointers to stl shared pointers (#162)

* Swiften 4 support
* using standard shared pointers when compiling with Swiften 4