sgsn: Shutdown PDP contexts cleanly on IMSI reuse (TODO)

Currently, if an IMSI is used by a newer MM context, the older MM
context referring to the same IMSI is freed without explicitly
deleting the established PDP contexts. This can lead to SGSN crashed.

This patch applies the full shutdown procedure like it is done when
receiving an explicit Detach Req to the old MM context to force a
proper cleanup of the PDP contexts before calling sgsn_mm_ctx_free.

TODO:
  - testing, unit tests

Addresses:
<0002> gprs_gmm.c:654 MM(/f6b31ab0) Deleting old MM Context for same
    IMSI p_tmsi_old=0xc6f19134
<000f> gprs_sgsn.c:259 PDP freeing PDP context that still has a
    libgtp handle attached to it, this shouldn't happen!
[...]
SEGFAULT

Ticket: OW#1310
Sponsored-by: On-Waves ehf