Remove the periodic timer and do rescheduling during Rx completion

Remove the periodic timer and do rescheduling during Rx completion

This change adds a check into the Rx ring completion routine that
schedules an interrupt task to be executed immediately after if
consumer index has already advanced itself.  The benefit of doing
this compared to an additional loop after replenishing the ring
(as done in FreeBSD for example) is that first of all this goes
through the loop in the taskqueue thread with a yeild check to
prevent CPU hogging and second is that it triggers Tx completion
as well since interrupt handler runs both.