Routing refactor

[Mallets]

Summary

In order to support more diverse scenarios, zenoh should be able to speak different routing protocols.
This would allow zenoh to efficiently route data on different types of networks and systems that are characterised by various level of stability, bandwidth, latency, and dynamicity.

Therefore, the code dealing with data routing in zenoh needs to be enhanced and modularised to accommodate the requirement of supporting multiple routing protocols.

This roadmap item will:

1. enable the new protocol (see issue Zenoh protocol update #2 ) to support diverse routing protocols on the wire;
2. allow a truly async API (see issue Truly async API #47 ) where routing code will no longer be traversed but queried.

Intended outcome

It will be possible to implement and configure multiple routing protocols in zenoh without the need of changing the core of the routing engine.

How will it work?

Users will be able to implement and configure multiple routing protocols in zenoh without the need of changing the core of the routing engine.