[lib] Workaround Flow issues in messages-reducer

Summary:
Part of ENG-3966

This is a controversial diff, the approach is discussed in comments.
Detailed description and all TODO follow-ups are tracked in ENG-4285. As soon as RN / Flow is upgraded, these should be reverted.

Depends on D8370

Test Plan:
This stack is tested altogether, CI will fail at this point due to Flow breaking changes: Some Flow issues are worked around by this diff, remaining ones are fixed in D8373
Actual test plan in the last diff.

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8371