Add EQC statem property for vclock. [rebased]

This is a QuickCheck property for vclock. The property was created so that the `dominates/2` could be added, with confidence that other parts of the API are solid as well. `dominates/2` essentially implements the `>` relation (strict-order) where `descends/2` implements the `>=` relation (partial-order). This could be used in various parts of `riak_kv` where vclocks are compared.

(h/t @joedevivo and @Quviq for their help on this)

/cc @russelldb