Fixed MongoDB retained messages

This PR addresses:

* Issue https://github.com/mcollina/mosca/issues/74 (Mongo persistence - multiple retained messages being received on subscribe). 
* Additional retained message unit tests included.

There are also some additional changes to get Mosca building, running and passing all unit tests on Windows:
* Bumped Microtime to 0.5.0 for Windows compatibility
* Moved Async_bench to optional depencies (can't build on Windows)
* Fixed "AfterEach" bug when running Unit tests on Windows
* SIGHUP unit test ignored when OS is Windows based.