[lib] refactor createOlmSessionWithPeer -> createOlmSessionsWithUser

Summary:
ENG-9106.

This API better matches Identity API, which returns data for all user devices.

Additionally, session creation for each device is done in parallel.

Test Plan: Tested in D13376

Reviewers: bartek, tomek, marcin

Reviewed By: bartek, tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13371