[merge] remove SFTPLock and support for weaves over SFTP

[Martin Pool]

This removes lock_write and lock_read on SFTPTransport so that they
just raise TransportNotPossible; as a consequence you can no longer
directly access weave branches (which are an extremely old format)
over SFTP.

-- 
Martin <http://launchpad.net/~mbp/>
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