- 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()

- cleanup: change grpc_iomgr_cb_func to take a bool instead of int
  success
- cleanup: follow through with iomgr callback scheduling functions
- prepare: add a workqueue to offload to to grpc_exec_ctx_enqueue*
  functions