use advisory locking on files being transferred

[msoulier]

Currently we do no locking at all. The client and server should use flock with
read/write locks if they are reading/writing, to prevent file corruption.

[msoulier]

On the client side this is simple, as we don't mind blocking. Although come to think of it, we might want to have an option to not-block on locking, but fail instead. The client might not want to block on locking.

And on the server it's trickier. We don't want to block at all. It would block the whole server.

[msoulier]

Scheduling for 1.0.