stats: use libosmocore rate counter for in/out_stream.err_ts_counter

The two counters: in_stream.err_ts_counter and out_stream.err_ts_counter
are still handcoded. To make them better accessible they should
 be replaced with libosmocore rate counters.

- replace state.in_stream.err_ts_counter with libosmocore rate counter
- replace state.out_stream.err_ts_counter with libosmocore rate counter

Change-Id: I9fbd65bf2f4d1e015a05996db4c1f7ff20be2c95
Related: OS#2517
diff --git a/include/osmocom/mgcp/mgcp_internal.h b/include/osmocom/mgcp/mgcp_internal.h
index 0da2c56..ff02768 100644
--- a/include/osmocom/mgcp/mgcp_internal.h
+++ b/include/osmocom/mgcp/mgcp_internal.h
@@ -28,6 +28,7 @@
 #include <osmocom/mgcp/mgcp.h>
 #include <osmocom/core/linuxlist.h>
 #include <osmocom/core/counter.h>
+#include <osmocom/core/rate_ctr.h>
 
 #define CI_UNUSED 0
 
@@ -45,7 +46,7 @@
 	uint32_t ssrc;
 	uint16_t last_seq;
 	uint32_t last_timestamp;
-	uint32_t err_ts_counter;
+	struct rate_ctr *err_ts_ctr;
 	int32_t last_tsdelta;
 	uint32_t last_arrival_time;
 };
@@ -202,6 +203,8 @@
 			uint32_t octets;
 		} stats;
 	} osmux;
+
+	struct rate_ctr_group *rate_ctr_group;
 };
 
 /*! Connection type, specifies which member of the union "u" in mgcp_conn