[statistics] Provide basic statistics for the NAT

Count number of SCCP connections, number of BSC reconnects,
number of calls. For most of them we have a per BSC and a
global count.

Right now all structs using the counters survive until the
end of the application so we do not need to free them.
diff --git a/openbsc/src/nat/bsc_sccp.c b/openbsc/src/nat/bsc_sccp.c
index 6e05a03..176f06a 100644
--- a/openbsc/src/nat/bsc_sccp.c
+++ b/openbsc/src/nat/bsc_sccp.c
@@ -100,6 +100,8 @@
 	}
 
 	llist_add(&conn->list_entry, &bsc->nat->sccp_connections);
+	counter_inc(bsc->cfg->stats.sccp.conn);
+	counter_inc(bsc->cfg->nat->stats.sccp.conn);
 
 	LOGP(DNAT, LOGL_DEBUG, "Created 0x%x <-> 0x%x mapping for con %p\n",
 	     sccp_src_ref_to_int(&conn->real_ref),