- make closures know where they should be executed (eg, on a workqueue,
  or a combiner, or on an exec_ctx)
- this allows removal of a large number of trampoline functions that
  were appearing whenever we used combiners, and should allow for a much
  easier interface to combiner locks

much metadata and refuses to send responses with too much metadata.

when the header includes a semicolon.

Initial interface rework to allow knowing whether to pull payload at registration, not at request time