mgcp: Move Annex A counting out of patch/count method

mgcp_patch_and_count has grown due supporting linearizing timestamps,
ssrc and other things for equipment like the ip.access nanoBTS. Fight
back and move the Annex A code into a dedicated method.

The result is updated as we now count after all the patching and for
the Annex A code no change in SSRC can be detected.
diff --git a/openbsc/include/openbsc/mgcp_internal.h b/openbsc/include/openbsc/mgcp_internal.h
index 3bccb39..0ec18cd 100644
--- a/openbsc/include/openbsc/mgcp_internal.h
+++ b/openbsc/include/openbsc/mgcp_internal.h
@@ -54,17 +54,22 @@
 
 	uint32_t orig_ssrc;
 
-	uint16_t base_seq;
 	int seq_offset;
-	int cycles;
 
 	int32_t  timestamp_offset;
 	uint32_t packet_duration;
-	uint32_t jitter;
-	int32_t transit;
 
 	struct mgcp_rtp_stream_state in_stream;
 	struct mgcp_rtp_stream_state out_stream;
+
+	/* jitter and packet loss calculation */
+	int stats_initialized;
+	uint16_t stats_base_seq;
+	uint16_t stats_max_seq;
+	uint32_t stats_ssrc;
+	uint32_t stats_jitter;
+	int32_t stats_transit;
+	int stats_cycles;
 };
 
 struct mgcp_rtp_codec {