[Direct Transfer] Introduce the notion of sessions

When adding a bunch of files/folders to a Direct Transfer, they should be tight to a session.

Plan:

• Add a new field in the States table: session, defaults to 0.
• When adding files to the database, via Engine._direct_transfer(): get the last session number + 1 for the session to be created.
• EngineDAO.plan_many_direct_transfer_items() should get a new argument: the session number.
• Add tests.