Merge adding lease with renewing lease (for immutables)

[itamarst]

1. From a client perspective, the goal is just to have a lease, whether it's a new lease or renewing a lease is irrelevant; renewing is essentially an optimization detail on server side.
2. Cancelling leases is not actually supported.
3. The way lease renewal secrets are generated is likely to change in the switchover from foolscap to HTTP.
4. Internally, the server _already_ treats renewing a lease as potentially adding a new one if renewal fails.

As such, the distinction between adding a lease and renewing a lease is unnecessary (presuming item 4 is actually correct!).

Therefore, we should merge the two into a single operation:

1. Validate item 4 above, since it's critical requirement for the rest of this.
2. In Foolscap client, switch to only using renewal code path.
3. Make sure server creating new lease when renewal fails is well tested, so users don't lose data.
4. On server-side, leave new lease Foolscap endpoint in place for backwards compat.
5. In new GBS HTTP protocol spec, switch to a single end point for lease creation/renewal, and remove all references to lease cancellation secret.