improve efficiency of propagating address failures

[dharjeezy]

closes #409

[lexnv]

I would add an explicit enum for this option:

pub enum DialFailureAddresses {
    Required,
    NotRequired,
}

Such that we can easily follow during the protocol registration the purpose of this flag

[lexnv]

We could introduce a tiny helper over the protocol context:

impl ProtocolContext {
    fn dial_failure(&self, addresses: &[Multiaddr]) -> Vec<Multiaddr> {
        match self.dial_failure_mode {
            DialFailureMode::Required => addresses.to_vec(),
            DialFailureMode::NotRequired => Vec::new(),
        }
    }
}

 let _ = match context.tx.try_send(
    InnerTransportEvent::DialFailure {
        peer,
        addresses: context.dial_failure(addresses)
    }
 )

This way we avoid cloning the addresses until we need them, while keeping the InnerTransportEvent::DialFailure event explicit. This will also provide a bit better isolation to avoid free floating functions

[lexnv]

nit: We could add some documentation explaining the purpose of this enum

[lexnv]

nit: Missing docs

[lexnv]

nit: Is there a way to avoid the cloning here? Maybe changing the signature of the fn to take a & Multiaddr?

I would also inline the make_dial_failure_event:

match context.tx.try_send(InnerTransportEvent::DialFailure { peer, addresses })

Instead of cloning the addreses here (addresses.clone()), we could recreate the addresses on error

[dharjeezy]

i have changed to this inline way, but can't still do without cloning