[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