Feature: add PayloadTooLarge error

[drmingdrmer]

Changelog

Feature: add PayloadTooLarge error

If a RaftNetwork implmentation found an AppendEntriesRequest is too
large, it could return a PayloadTooLarge::new_entries_hint(n) error to
tell openraft devide request into smaller chunks containing at most n
entries. Openraft will limit the number of entries in the next 10
AppendEntrie RPC.

Exmaple:

impl<C: RaftTypeConfig> RaftNetwork<C> for MyNetwork {
    fn append_entries(&self,
            rpc: AppendEntriesRequest<C>,
            option: RPCOption
    ) -> Result<_, RPCError<C::NodeId, C::Node, RaftError<C::NodeId>>> {
        if rpc.entries.len() > 10 {
            return Err(PayloadTooLarge::new_entries_hint(10).into());
        }
        // ...
    }
}

---

This change is