pipelined calls sometimes return unimplemented

[zenhack]

Sometimes making a pipelined call on a result will spuriously return unimplemented, without invoking the actual object. @lthibault and @aratz-lasa report that they've seen this from time to time, and it seems to be the root of zenhack/tempest#11. Waiting on the result seems to work around the problem consistently.

[lthibault]

Related to note in Matrix by @zenhack?

Note to self: I think the remaining ordering bugs are a function of continuing to use Clients in some places where we should have switched over to ClientSnapshot