Fixed interaction between paxos and finalization protocol

Now acceptor ignores any paxos messages for finalized instances. Since
learner already delivered a value, all further paxos rounds can not
change that value without breaking the safety property.
This fix should stop extra values from appearing in the test run.

Also acceptor now sends special flag `final' for finalized instances, which
makes learner ignore paxos logic for majority element and closes the
instance.
This should fix paxos halting when network is unstable.