Make TCP shutdown more conditional

[bz]

Summary

Check the V_tcbinfo.ipi_count to hit 0 before doing the full TCP cleanup.
That way timers can finish cleanly and we do not gamble with a DELAY().

Note: we still sleep initially for code simplicity. VNET teardown is a slow path;
if someone wants to move this and make the pause conditional on the value of n, then feel free.
I found the tcpdes pause to be helpful when looking at ps (where I regularly saw it).

[jtl]

Looks like a great change. Thanks for your work!