The underlying bus was thread_local, but the design assumption that Variants built on top of that instance won't outlive the thread was incorrect. In stress tests, Variants were moved (and this is completely legal) to a different thread.
Introduce mock of sdbus library through extracting its interface. Set up unit tests of Connection class through injection of sdbusMock to constructor. Clients of Connections class should use fabrics instead.
* Add preliminary changes for async server methods
* Refactor the Message concept and break it into distinctive types
* Continue working on async server methods (high-level API mainly)
* Continue developing support for async server
* Finishing async server methods
* Finishing async server methods (fixing tests & cleaning up)
* A little code cleaning
* Add unit tests for type traits of free functions
* Support for generating async server methods in stub headers
* Update ChangeLog for v0.3.0
* Update the tutorial with how to use async server-side methods
* Update the TOC in sdbus-c++ tutorial
* Update numbering in TOC
* Remove unnecessary code
* Final cleanups
