Ts versions and syntonization

Syntonization was not working for me on an i210 nic. 
I figured out it was due to an invalid timestamp introduced in the addition operator (Timestamp class).
This pull request introduces a constructor with version parameter. 
Addition and subtraction functions use this new signature to assign the operand's ts version.
With this fix syntonization works for me.

Anyhow, I don't really get where these version attribute stems from. Can someone shed a light on this for me?

Best Regards
Marc 

P.S. of course this pull request should be applied against open-avb-next. Github messed that up somehow...