tbf: Set MS during constructor time

This is another step forward towards a more clear data model where a TBF
always has a MS object (which may be lacking some information, and at a
later point when more information is found, it may actually be a
duplicated MS object and hence one duplicate removed and the TBF moved
to the object being kept).

This helps for instance in removing duplicated information stored in
the TBF which is really per MS, like ms_class, ta, etc. Since there's
always a MS object there's no need to keep a duplicate in both classes
in case there's no MS object.

It can already be seen looking at unit test logging that this kind of
data model already provides better information.
Some unit test parts were needed to adapt to the new model too.

Change-Id: I3cdf4d53e222777d5a2bf4c5aad3a7414105f14c
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index cbefb1b..0d692cc 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -60,7 +60,7 @@
 					const uint8_t ms_class)
 {
 	if (!tbf->ms_class() && ms_class)
-		tbf->set_ms_class(ms_class);
+		tbf->ms()->set_ms_class(ms_class);
 }
 
 static void llc_timer_cb(void *_tbf)
@@ -85,8 +85,8 @@
 	timespecclear(&dl_loss_tv);
 }
 
-gprs_rlcmac_dl_tbf::gprs_rlcmac_dl_tbf(BTS *bts_) :
-	gprs_rlcmac_tbf(bts_, GPRS_RLCMAC_DL_TBF),
+gprs_rlcmac_dl_tbf::gprs_rlcmac_dl_tbf(BTS *bts_, GprsMs *ms) :
+	gprs_rlcmac_tbf(bts_, ms, GPRS_RLCMAC_DL_TBF),
 	m_tx_counter(0),
 	m_wait_confirm(0),
 	m_dl_ack_requested(false),