[lib] improve processOutboundP2PMessages in terms of performance

Summary:
ENG-9096

We need to do it per device, not per message to avoid creating multiple sessions in parallel.

Test Plan: Test DM ops (text messages, settings, reactions, etc.)

Reviewers: tomek, marcin

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13204