Function raftstore::store::util::is_first_append_entry

fn is_first_append_entry(msg: &Message) -> bool

is_first_append_entry checks msg is the first append message or not. This meassge is the first message that the learner peers of the new split region will receive from the leader. It’s used for when the message is received but there is no such region in Store::region_peers. In this case we should put msg into pending_msg instead of create the peer.