MINOR: proxy: add 'served' field to proxy, equal to total of all servers'

This will allow lb_chash to determine the total active sessions for a
proxy without any computation.

Signed-off-by: Andrew Rodland <andrewr@vimeo.com>
diff --git a/src/stream.c b/src/stream.c
index 151bcb0..738a23c 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -2515,6 +2515,7 @@
 
 	if (sess->srv_conn) {
 		sess->srv_conn->served--;
+		sess->srv_conn->proxy->served--;
 		if (sess->srv_conn->proxy->lbprm.server_drop_conn)
 			sess->srv_conn->proxy->lbprm.server_drop_conn(sess->srv_conn);
 		stream_del_srv_conn(sess);
@@ -2522,6 +2523,7 @@
 
 	if (newsrv) {
 		newsrv->served++;
+		newsrv->proxy->served++;
 		if (newsrv->proxy->lbprm.server_take_conn)
 			newsrv->proxy->lbprm.server_take_conn(newsrv);
 		stream_add_srv_conn(sess, newsrv);