vault() opened, named by its
id. The wallet fetches the prepared cancel, sends it with the cost covered by
Endstate, and resolves once the network has included it. The request reads
cancelled once Endstate sees it. Only the wallet that approved the request
can send it.
Rejects with submission.not_executable when the request is already closed,
with vault.request_not_found for an unknown id, with auth.forbidden when
this wallet did not approve the request, with wallet.sponsorship_unavailable
when the send cannot be covered right now (nothing was sent), and with
wallet.destroyed if the wallet has been destroyed.

