use configurable persistent connection in upstream health check

1) add directive "check_keepalive_requests" which controls the request number of a persistent connection its default value is 100.
2) for ssl, mysql and ajp protocol, persistent connection is not used, in this case, directive "check_keepalive_requests" does not work.
3) for http protocol, if the status code is 4xx or 5xx, persistent connection will be closed.
4) The health checking request must contain header “Connection: keep-alive” to announce using persistent connection, or else the upstream server will close the connection.