[lib] update processOutboundP2PMessages to return IDs of messages queued on Tunnelbroker

Summary:
ENG-8423.

To implement retries, we need to track which P2P messages are sent, which are not.

This code is bad in terms of performance - for each deviceID we can create a Promise and avoid awaiting in sequence, this will be improved as a follow-up.

Depends on D13149

Test Plan:
Await promise and check if IDs are returned.

Note: This stack is tested end-to-end according to the process described here.

Reviewers: tomek, inka

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13150