Improve collator side of the collator-protocol (#1955)

* Improve collator side of the collator-protocol

This pr improves the collator-protocol implementation of the collator
side. Besides cleaning up code and rewriting it, the following changed:

- Before on `PeerViewChange` we send an advertisment to every peer, now
this only happens for validators.
- It also adds a check that we send an advertisment message only once
for a connected peer.
- If the same validator was part of the current and next group, we
requested to be connected to this validator two times. This is also
fixed now.
- Instead of having only one connection request, we now are being able
to store multiple of them. This is required as we can have multiple
active leafs at any point of time.

* Switch to common `ConnectionRequests`

* Update node/network/collator-protocol/src/collator_side.rs

Author: bkchr