[merge] remove SFTPLock and support for weaves over SFTP

[John Arbash Meinel]

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Martin Pool wrote:
> 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.
> 
> 

So IIRC, Weave repos used an OS lock, but for SFTP access we created a
file with O_EXCL. Which meant that locking over SFTP was not actually
lock-safe versus local access. (It was safe versus another SFTP access,
which was, in general, good enough, but we went a different route to be
truly safe against all forms of access.)

So I'm happy to remove it, though it might be nice to have a good error
of "cannot access Weave repositories over sftp" somewhere.

What brought this up, btw?

John
=:->

BB:approve
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