Use DI and mocking so we have code coverage of the SessionManager
Improve test commenting
Fix build error
Update cpp-common
Merge pull request #5 from Metaswitch/submodule_update

[No reviewer] Update cpp-common
Merge pull request #6 from Metaswitch/coverage

[Reviewer: Andy] Use DI and mocking so we have code coverage of the SessionManager
Merge pull request #7 from Metaswitch/coverage

[No review required] Fix build error