Reapply r223198 which was reverted in the previous vendor import. Some

Reapply r223198 which was reverted in the previous vendor import.  Some
portions were already reapplied in r233708:
- Use a dedicated task to handle deferred transmits from the if_transmit
  method instead of reusing the existing per-queue interrupt task.
  Reusing the per-queue interrupt task could result in both an interrupt
  thread and the taskqueue thread trying to handle received packets on a
  single queue resulting in out-of-order packet processing.
- Call ether_ifdetach() earlier in igb_detach().
- Drain tasks and free taskqueues during igb_detach().

MFC after:	1 week