Currently, the TCP slow timer can starve TCP input processing while it
walks the list of connections in TIME_WAIT closing expired connections
due to contention on the global TCP pcbinfo lock.

To remediate, introduce a new global lock to protect the list of
connections in TIME_WAIT.  Only acquire the TCP pcbinfo lock when
closing an expired connection.  This limits the window of time when
TCP input processing is stopped to the amount of time needed to close
a single connection.

Submitted by:	Julien Charbon <jcharbon@verisign.com>
Reviewed by:	rwatson, rrs, adrian
MFC after:	2 months