Remove orphaned transfers

Issue

When resuming or pausing a transfer, this pseudo-code is used:

engine = manager.engines.get(engine_uid)
if not engine:
    return
engine.resume_transfer(transfer_type, transfer_id)

But if, for one reason or another, the engine associated does not exist anymore, the transfer is orphaned.

Fix

• Remove the transfer from the database.
• If the transfer is a download, then we should also remove the eventual partially downloaded file.