Add some helpers for substream upgrades

[tomaka]

cc #833

Adds the write_one, read_one and request_response methods in upgrade. Makes it possible to easily write one message, read one message, or write then read one message.
Makes floodsub use these new methods.

Eventually I'll change Kademlia to also use them (cc #830).
Ping is blocked by #884. Either this PR should be rebased over #884, or the opposite.

[tomaka]

Ping is blocked by #884.

Actually, since the Ping protocol doesn't have any length prefix, we can't use these helpers.

[twittner]

Suggested change

	let mut len_data = [0; 10];
	let mut len_data = unsigned_varint::encode::u64_buffer();

[twittner]

How about:

let n = cmp::min(data_start.len(), len);
let mut data_buf = vec![0; len];
data_buf[.. n].copy_from_slice(&data_start[.. n]);

[twittner]

Instead of taking a TThen parameter, why could the ReadOne future not just yield the Vec<u8> and the existing future combinators are used?

The Floodsub protocol implementation would be

upgrade::read_one(socket, 2048).from_err().and_then(|packet| { ... })

instead of

 upgrade::read_one(socket, 2048, |packet| { ... })

Granted, the associated FloodsubConfig::Future type is somewhat more difficult with

 type Future =
    AndThen<
        FromErr<upgrade::ReadOne<TSocket>, FloodsubDecodeError>,
        Result<FloodsubRpc, FloodsubDecodeError>,
        fn(Vec<u8>) -> Result<FloodsubRpc, FloodsubDecodeError>
    >;

But that just reflects the state of the current futures system and if performance is less of an issue one could always use trait objects instead.

[tomaka]

I'll add both versions.