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/src/libosmo-mgcp/mgcp_network.c b/src/libosmo-mgcp/mgcp_network.c
index 49e51a1..4144382 100644
--- a/src/libosmo-mgcp/mgcp_network.c
+++ b/src/libosmo-mgcp/mgcp_network.c
@@ -222,7 +222,7 @@
 
 	if (seq == sstate->last_seq) {
 		if (timestamp != sstate->last_timestamp) {
-			sstate->err_ts_counter += 1;
+			rate_ctr_inc(sstate->err_ts_ctr);
 			LOGP(DRTP, LOGL_ERROR,
 			     "The %s timestamp delta is != 0 but the sequence "
 			     "number %d is the same, "
@@ -272,7 +272,7 @@
 	    ts_alignment_error(sstate, state->packet_duration, timestamp);
 
 	if (timestamp_error) {
-		sstate->err_ts_counter += 1;
+		rate_ctr_inc(sstate->err_ts_ctr);
 		LOGP(DRTP, LOGL_NOTICE,
 		     "The %s timestamp has an alignment error of %d "
 		     "on 0x%x SSRC: %u "