[lib] allow to overwrite content session

Summary:
ENG-6982.

When discovering session is malformed we want to create a new one (and resend messages, implemented later in the stack).

initializeContentOutboundSession implementation allow to override session by default.

Test Plan: Being able to create new session even if one already exists

Reviewers: marcin, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13024