test(instance_test.go) add tests to cover prepare-reply helpers

add TestUpdateReocveryInstance
add TestHandleCommittedPrepareReply
add TestHandleAcceptedPrepareReply
add TestHandlePreAcceptedPrepareReply
chore(instance.go) handleAcceptedPrepareReply

Instead of panic we should just return if
recoverInfo.ballot is equal to the ballot of the prepare-reply.
See issue #45
chore(instance.go) handlePreAcceptedPrepareReply

Omit setting identicalCount to zero, We can just leave the identicalCount
not changed. see issue #45