don't do lease-renewal automatically

[zooko]

Currently lease-renewal happens when you explicitly tell your client to create a file or to renew leases, and also on some other occasions. I think that the "other occasions" are only when you update a mutable file, but I'm not sure. I don't think this is appropriate because in the future you might want to update a file without thereby agreeing to pay for the file's preservation.

To close this ticket:

1. Read the source code and write down on this ticket all the ways that leases can get added or renewed.
2. Write a patch that removes the lease renewal from all cases except for when the client is either creating a file or sending an "Add or Renew Lease" request.
3. Of course, unit tests of step 2 are required!

Brian: please let us know your opinion of this.