lightning-net-tokio: Allow custom smart pointers

[MaxFangX]

Summary

This PR is simple: for the APIs in lightning-net-tokio, instead of requiring that the PeerManager's ChannelMessageHandler, RoutingMessageHandler etc are exactly an Arc<Trait>, anything that impls Deref where Deref::Target: Trait suffices.

Motivation

In our project we are using newtypes (e.g. LexeKeysManager) which Deref to the LDK equivalent (KeysManager) which implement the important traits (KeysInterface). We've found that using custom smart pointers in this way is a convenient way for us to implement custom methods and traits on the newtype (e.g. init() -> Self, TryFrom, derive_pubkey()) while reusing the default trait implementations provided by LDK. Since many of LDK's APIs are already generic across Deref impls, this PR is essentially extending that idiom to lightning-net-tokio.

API Changes

Since all calling code currently uses Arc<Trait>, and Arc is Deref::Target: Trait, this PR introduces 0 API-breaking changes.

Implementation Notes

The 'static + Send + Sync has to apply to both the generic argument (e.g. CMH) as well as its target (e.g. CMH::Target). The ?Sized marker was removed from Logger, as Sized is required.

I have integrated this change in our own project and it's working perfectly as a "drop in" replacement of the existing lightning-net-tokio API.

[TheBlueMatt]

Fixes #1536