LockableFiles.__del__ creates uncollectable garbage including connected sockets

Running this code:

>>> while 1:
... b = bzrlib.branch.Branch.open("http://localhost:8080/trunk")
... del b
... gc.collect()

will eventually run you out of file descriptors. Looking in gc.garbage reveals lots of socket._fileobject objects. I'm not _completely_ sure why, but commenting out LockableFiles.__del__ makes this problem go away, so we should try to find a way of retaining its functionality without an actual __del__ method on the class itself.