fix: unify which term is used for cas predicates

CAS predicates should always contain the current raft term, because this
is what's explicitly checked in proc_cas.

Note that this will sometimes result in a EntryTermMismatch error
because the latest applied entry may have a different term in case the
election has started but not finished yet.

We could add this check to all the client cas precondition checks, but
it's not a big deal, because we handle the situation correctly anyways.