ms: Merge ms_storage into bts.cpp

That class is mostly a C++ class holding a llist plus some callbacks.
Having that in a separate class makes code more complex for no good
reason. This patch moves the llist into bts and allocates stuff directly
from within bts.
This will allow further cleanup of MS lieficyle in future patches.

Change-Id: I627f5db5073189c23ddf2b7f09c90abb24846f62
diff --git a/src/gprs_bssgp_rim.c b/src/gprs_bssgp_rim.c
index 033426b..aa636b6 100644
--- a/src/gprs_bssgp_rim.c
+++ b/src/gprs_bssgp_rim.c
@@ -152,7 +152,7 @@
 	}
 	entry = si_cache_add(bts->pcu->si_cache, &nacc->reprt_cell, &val);
 
-	llist_for_each(tmp, bts_ms_list(bts)) {
+	llist_for_each(tmp, &bts->ms_list) {
 		struct GprsMs *ms = llist_entry(tmp, typeof(*ms), list);
 		if (ms->nacc && nacc_fsm_is_waiting_si_resolution(ms->nacc, &nacc->reprt_cell))
 			osmo_fsm_inst_dispatch(ms->nacc->fi, NACC_EV_RX_SI, entry);