Currently only TBF objects are used to handle the data flow between
the MS and the SGSN. MS specific data (e.g. pending LLC frames, TLLI)
is copied between successive TBFs. If all TBFs (uplink and downlink)
are idle for some time, all information about the MS is discarded in
the PCU. This makes the implementation of some features more
difficult, e.g. proper TLLI and timing advance handling,
connection based CS selection, and proper management of multiple TBF.

This commit adds the GprsMs class that is intended to hold
information directly related to the MS and to keep references to the
active TBFs.

The class is not yet integrated with the other PCU code. A GprsMs
object container and MS specific fields (TA, CS) will be added in
later commits.

Note that calling detach_tbf() can possibly delete the MS object
depending on the callback implementation.

Ticket: #1674
Sponsored-by: On-Waves ehf