[lib] Stop processOutboundMessages loop when it becomes outdated

Summary:
This addresses ENG-9454 and ENG-9448.

When we update this callback, we should make sure we stop running old versions of the callback. Otherwise, there's a risk that we'll process new data with callbacks that are bound to old versions of the Redux state.

Test Plan:
I had a repro of ENG-9448 where I simply logged out and back in with my t125 test user. After this diff:

1. I no longer see Cannot read properties of undefined errors
2. The robotext messages are delivered for all of my Farcaster friends

Reviewers: varun, will, tomek, kamil

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13563