gbproxy: Reset TLLIs when the link_info is found by IMSI/PTMSI (TODO)

TODO:
  - commit message
  - fix existing test case (breaks ATM)
  - add test with Attach Req(IMSI) and an existing link_info with
    bogus non-zero TLLIs

This issue triggers an SGSN issue that leads to leaking LLMEs.

Ticket: OW#1324
Sponsored-by: On-Waves ehf
sgsn: authorization and other bugfixes (WIP)

TODO:
  - split this into several commits
  - ...

Sponsored-by: On-Waves ehf