- 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

- 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

appropriate

Rename ThreadPool -> FixedSizeThreadPool to allow for later introduction
of DynamicSizeThreadPool

This reverts commit 5d5b1d8c8ec4c578f72a12f5b81f705764cdf804.