add GlobalInfoServlet http header Content-Length check.

in client use, ServerDiscover must check this url , to avoid
bad  server  attack ,this response must has a Content-Length header
server discover will limit his value, to avoid eat too much resource
update TODO
add ServerDiscover.ScanCallback.onFinished to notify all ip checked
add some comment
add comment to ServletTestBase
ServerDiscover verbose more
override GlobalInfo.toString
import cleanup
add ScanCallback.onScan percent parameter