ms: Add MS storage class

Currently the MS objects are contained in the TBF objects only. To
allow for an extended life time after the TBF objects have been freed
and to find them based on TLLI, a container for the MS objects is
needed.

This commit adds the container class and also adds the corresponding
m_list member to GprsMs. Further integration into the PCU code is not
yet done.

Ticket: #1674
Sponsored-by: On-Waves ehf
diff --git a/tests/ms/MsTest.ok b/tests/ms/MsTest.ok
index a0d4e9e..219eec1 100644
--- a/tests/ms/MsTest.ok
+++ b/tests/ms/MsTest.ok
@@ -8,3 +8,5 @@
   ms_active() was called
   ms_idle() was called
 === end test_ms_replace_tbf ===
+=== start test_ms_storage ===
+=== end test_ms_storage ===