Allow event timestamps to be overriden
Tweak testcase name
Do not allow timestamps on markers to be overriden
Merge pull request #42 from Metaswitch/timestamp_override

[Reviewer: Jim] Timestamp override
Remove virtual methods from Message to prevent clearwater UT breakages
Merge pull request #43 from Metaswitch/get_timestamp_refactor

Remove virtual methods from Message to prevent clearwater UT breakages