tor-rtmock: Introduce MockTimeCore
(from one of the commit messages)
It bothered me in !2050 (merged) that when I added a MockCoarseTimeProvider
to the mock time providers, I had to manually grep to check that there
weren't places the time was updated where I ought to also update the
coarse time.
Prove that the code is right by hiding all the times together in a struct that prevents un-synchronised updates.