Add the client side of the backup API.

This builds on the work of Michael Foord.  I've reorganized the code a little, added tests and factored out the raw RPC code from the backup client code.  (The raw RPC can later be re-used by the other 2 client methods that currently duplicate the code.)
I thought doing a backup saves a copy of the backup on the server (on every server in HA for that matter) in addition to sending a copy to the client?