Support multiple responses for a single request (e.g. send full-duplex info packets)

[eeelllccc]

we have

pub enum HalfDuplexResponse {
    SocketError,
    Payload(Vec<u8>),
}

#[derive(Debug)]
pub struct HalfDuplexExchange {
    pub request: Vec<u8>,
    pub response: HalfDuplexResponse,
}

need to add

pub enum FullDuplexResponse {
    SocketError,
    Payloads(VecDeque<Vec<u8>>),
}

#[derive(Debug)]
pub struct FullDuplexExchange {
    pub request: Vec<u8>,
    pub response: FullDuplexResponse,
}

then RecvBuffer probably becomes:

pub trait Exchange {
    fn write_to_recv_buffer(self)
}

#[derive(Debug)]
pub struct RecvBuffer<T: Exchange> {
    pub queue: Mutex<Option<T>>,
    pub cv: Condvar,
}