Mfp: r296310,r296343
It looks like as with the safety belt of DELAY(hz) fastened (*) we can
completely tear down and free all memory for TCP (after r281599).
(*) in theory a few ticks should be good enough to make sure the timers
are all really gone. I wonder if we could use a better matric here and
check a tcbcb count as an optimization.
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