Use `add_lease` instead of `renew_lease` everywhere.

Created by: tomprince

As far as I can tell, add_lease will add or renew a lease as appropriate, but renew_lease will fail if a lease with our renewal secret does not exist.

I think we should:

• switch lease_maintence to call add_lease
• make remote_renew_lease delegate to remote_add_lease in our server plugin

This was also suggested upstream in https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3773