Solve head of line blocking

[povilasb]

The Problem

Currently the maximum message size Crust allows to send is 2 MB. The limit is there for a reason: we don't want upper layers to put too big TCP packets on the stream because it would simply clog the line. Basically this is head-of-line blocking problem. Crust users might and should not be aware of it - we strive for simplicity.

Design the changes so that Service::send(msg_100mb) would not delay Service::send(msg_10kb) delivery for too long.

Solution

1. Crust should fragment the data it's given before sending over TCP.
2. Then on the other side packets should be defragmented, buffered and proper crust events dispatched when a full packet arrives.
3. See if this multiplexer library could help: https://github.com/paritytech/yamux

[douglascaetano]

Closed in favor of #1145.