time: Implement ConvertableTimestamp::convert()

time: Implement ConvertableTimestamp::convert()

* This method is analogous to wfTimestamp(). Optimise for the common
idiom of just converting a timestamp without having the caller hold
on to any object.

* Make wfTimestamp() use this (it could already since it didn't
use any MWTimestamp methods). Use via MWTimestamp. While this is
the same as direct access, it allows future changes.

* Add tests covering this new method.

Change-Id: I7f9104f1701d92fe25d72c7943581c64e1d093fa