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.