client-side observations don't support renewal

[chrysn]

observe-16 notes that clients can send a request inside an observation with the same token to "to reinforce [their] interest in a resource".

without such a mechanism, clients will wait indefinitely for updates when the server goes away.

a first implementation should allow the requester to trigger such an action; when that is done we could think about having this done in a configurable interval starting after the last message.

the server side of this is currently untested, and should receive proper testing once the client can do that.