[lib] update processOutboundP2PMessages return type

Summary:
ENG-8424.

Addresses https://phab.comm.dev/D13325#inline-76680.

Depends on D13332

Test Plan: Repeat test plan from D13325

Reviewers: tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13351