tbf: Add BTS::ms_alloc method

Currently the code that creates the MS objects with tbf.cpp is
duplicated.

This commit moves the corresponding code into a new method. Since
there is no TLLI available there, the GprsMsStorage::create_ms method
has been refactored into two variants: one with TLLI/direction and
one without.

Sponsored-by: On-Waves ehf
diff --git a/src/bts.cpp b/src/bts.cpp
index 0ff0ffa..bc5ac94 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -522,6 +522,17 @@
 }
 
 
+GprsMs *BTS::ms_alloc(uint8_t ms_class)
+{
+	GprsMs *ms;
+	ms = ms_store().create_ms();
+
+	ms->set_timeout(m_bts.ms_idle_sec);
+	ms->set_ms_class(ms_class);
+
+	return ms;
+}
+
 /*
  * PDCH code below. TODO: move to a separate file
  */