Function serve 

pub fn serve<H>(
    host: &H,
    caller: &Caller,
    message: RemoteActionMessage,
) -> Option<Pending<RemoteActionMessage>>
where
    H: ActionHost + Clone + Send + 'static,

Answer one request from caller against host.

Returns None for a message that is not a request (a response arriving at a host is dropped, never answered). A refused invocation becomes a ReceiptStatus::Rejected receipt so the caller always learns the fate of the id it chose; an answer the host abandons becomes a refusal too.

A request that asks to wait (wait_ms on RemoteActionMessage::Observe or InvokeRequest) is answered once the invocation moves, through ActionHost::next_change and without holding a thread; see settle_with. A host that cannot signal changes answers at once.