ms: Move MS information merging to GprsMS

Currently the merging of the meta information (MS class, IMSI) takes
place in gprs_rlcmac_tbf::merge_and_clear_ms(). This makes it
difficult to merge the internal state and does not directly relate to
TBFs anyway.

This commit moves this into a new method GprsMs::merge_old_ms.

Sponsored-by: On-Waves ehf
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index 1bc08b7..a0510f6 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -230,7 +230,7 @@
 			if (ms_old->dl_tbf() && ms_old->dl_tbf()->T == 0)
 				tbf_free(ms_old->dl_tbf());
 
-			ms_old->reset();
+			ms->merge_old_ms(ms_old);
 		}
 	}