Move AbstractTransport/TransportInterface from thruway/client to voryx/thruway-common

[boenrobot]

I wasn't sure if this is the right place, as this is a cross repo organization issue, but seeing this repo is the final destination, it's probably the right place...

If one tries to open a thruway router transport like thruway/ratchet-transport as a standalone project, and install it, they don't have all the dependencies required to get the full picture. Specifically, the abstract transport is not present. The project thruway/client needs to be added to that composer.json's require-dev to address this. However, that doesn't sound right to me for a router transport.

So I think the class \Thruway\Transport\AbstractTransport and the interface \Thruway\Transport\TransportInterface should be moved to voryx/thruway-common and "voryx/thruway-common": "*" should be added as require-dev to all transports, including thruway/ratchet-transport.