Fix double free in DummyConnectionServer

Do not create shared ptr from this as this lead to double free in
UserRegistryTest::login test. Shared ptr was needed to set event
owner in acceptConnection, actually it is never needed as events
are never filtered by owner. Thus it was removed and there is no
need to create shared ptr from this.

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