fix bug:

When one worker process is killed, it should be removed from master's children_
list

bug example:

./cwfd -F 1 -t 1
master pid=1200
worker pid=1201

$ kill -s SIGKILL 1201

then master fork new worker process whose pid is 1202

if now, I shutdown cwfd
$ kill -s SIGTERM 1200

the worker prcess is still alive!