gbproxy: Remove the global rate counter and place it in the config

Move the global data into the struct and use it. gbprox_reset will
first free data and then re-initialize the structure. This code is
used by the unit test.

Done with Jacob
diff --git a/openbsc/src/gprs/gb_proxy_vty.c b/openbsc/src/gprs/gb_proxy_vty.c
index 2ca0c57..ee13e4c 100644
--- a/openbsc/src/gprs/gb_proxy_vty.c
+++ b/openbsc/src/gprs/gb_proxy_vty.c
@@ -341,7 +341,7 @@
 	int show_stats = argc >= 1;
 
 	if (show_stats)
-		vty_out_rate_ctr_group(vty, "", get_global_ctrg());
+		vty_out_rate_ctr_group(vty, "", gbcfg.ctrg);
 
 	llist_for_each_entry(peer, &gbcfg.bts_peers, list) {
 		gbprox_vty_print_peer(vty, peer);