Revert "Use sender instead of providing actor refs (#1379)"

[pm47]

It was error-prone (cf bff833b) and not a good practice according to @sstone.

Commit 2ad91b2 is optional, let me know what you think.

[t-bast]

I like the revert, but not sure about 2ad91b2
While it simplifies a bit the code, PeerRoutingMessages are not necessarily gossip, so if we want to make those evolve independently (add new fields) we'll likely need to revert that.
Not a strong NACK, but not a strong ACK either :)

[pm47]

I like the revert, but not sure about 2ad91b2
While it simplifies a bit the code, PeerRoutingMessages are not necessarily gossip, so if we want to make those evolve independently (add new fields) we'll likely need to revert that.
Not a strong NACK, but not a strong ACK either :)

I reverted with 13ea541 (insert 'yo_dawg.jpg') but kept the LightningMessage->RoutingMessage signature change.