This reverts commit 5e01e2ac.

calls/channels

Use it to ensure we don't delete the call from that thread: doing so
would create a cycle that's kind of bad.

- 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

Allows exec_ctx callbacks to be called while a combiner is executing.
Also allows guaranteeing direct execution of callbacks from combiners,
which should allow reducing cpu burn for up/down stack interactions in
the future.

Delay beginning most writes until we enter poll()