[tcp-scale]: A connection in TIME_WAIT state before
calling close() did not actually received any RST.
Don't set error to ECONNRESET in this case.
[tcp-scale]: Introduce the INP_LIST global mutex for protecting pcbinfo
global structures. Then use INP_INFO_RLOCK in critical paths to increase
TCP processing parallelism, and use INP_INFO_WLOCK in full INPs iteration
loops.

Julien's review:
 - Fix INP_INFO_WLOCK assertions
 - Improve comments