By review the processing of InactivityCop::check_inactivity():
1. get all local vc from open_list
2. put them into cop_list
3. check every vc in cop_list if it is already timeouted
4. callback vc->handleEvent to close vc if it is timeout
InactivityCop and NetHandler share one mutex.
InactivityCop runs every second, NetHandler runs every 10ms, that means Nethandler runs 100 times until next InactivityCop runs.
if one vc has read/write in a Nethandler call, it is won't be timeout in the next InactivityCop run.
Thus, if the vc has read/write in Nethandler, we move it out of cop-list then the InactivityCop runs would get better performace.