Tried removing "session = Session()" line from loadable.py (in merlin and arthur) to reduce postgres idle processes. The "session" is actually quite self-contained, so I've put it back, but removed references to Core.db's session.

Signed-off-by: Martin Stone <martin@d7415.co.uk>
Arthur's loadable.py wasn't closing the new session.

Signed-off-by: Martin Stone <martin@d7415.co.uk>