Allows diagnosing WHY a file descriptor was shutdown prematurely.

- 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

Localize global_wakeup_fd declarations, instead of trying to share them,
so that this bug is less likely to occur in the future.

Delay beginning most writes until we enter poll()