clockvector: fix 'happens_before', change name to 'synchronized_since'

clockvector: fix 'happens_before', change name to 'synchronized_since'

First, the 'happens_before' function needed to use an inclusive 'less-than'
(i.e., <=) instead of 'strictly less-than' (i.e., <) in order to fit with
most of the algorithm.

Second, the association of 'happens_before' directly with the ClockVector is
not very intuitive to use. I will write an additional happens_before()
function that can easily compare two ModelAction objects. So change this
happens_before() (with it awkward usage) to be called synchronized_since(),
representing whether the ClockVector's thread has previously synchronized
with the ModelAction's thread.