Added RELEARN message type

Semantics are same as LEARN, but for statistical purposes LEARNs sent
in response to RETRANSMIT are now called RELEARN and are logged
accordingly to the perflog.
Check of ballot number in short-circuit for acceptor

Not it makes sure that ballot is 1 when it receives an accept for an
instance it knows nothing about.

Also added a reject in case of instance has never ballot than the one
proposer is proposing.