Start the session on rest.php

Trust me, I lost hours and hours on that one: civi needs a session otherwise it's going to fail in random ways at random places (eg. if you add an employer on a contact create AND you want to add a related phone chaining the api it will fail creating the api)

session_start() seems to be the way it's done in various other places (in external or bin). Is this the proper way?
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