action, clockvector: add 'has_synchronized_with()' functions

action, clockvector: add 'has_synchronized_with()' functions

These functions will check whether a ClockVector (or corresponding ModelAction)
is *completely* synchronized with another already. This is different from
simply "happens before," because I may need to update and propagate a clock
vector after initial execution as more information becomes available, and so
this function helps determine whether a particular pair of vectors is worth
merging (and then - expensively - propagating) before actually performing the
synchronization.

[Not documented properly yet...]