[lib] convert peerToPeerMessageHandler to a hook

Summary:
Avoids passing a lot of callbacks as arguments and makes code easier to maintain.

Depends on D12371

Test Plan: Flow - this is just moving code to a hook, logic remains unchanged. Functionality tested later in the stack.

Reviewers: bartek, tomek

Reviewed By: bartek, tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12372