Handle PDU Sessions in NG Initial UE context Setup and forward 5G-S-TMSI to core

• store 5G-S-TMSI when the UE signals this to RRC, and forward via NGAP to core
  • removes assert that stopped the gNB upon receiving it
  • fix NGAP 5G-S-TMSI encoding, forward 5G-S-TMSI to core
• Handle PDU sessions in NG Initial UE context setup - resolves #672 (closed)
• Experimentally add the UE retention information in the NG setup request (now in commented form). OAI 5GC v1.5.1 and v2.0 fail to decode NG Setup request with UE retention information. there is a fix in v2.0.1, let's wait a bit before merging this (at least one month to give people time to upgrade) ue-ret-info.patch
  • Need to test with v2.0 and see if the behavior is the same; if yes, we might not be able to include this as long as there is no fixed official release of OAI 5GC
• Implement reestablishment handling from a new DU (i.e., not where the UE was connected previously). The implementation is simplified: in fact, we trigger connection setup, instead of proceeding with a reestablishment.
• Fixes/Cleanup in various places:
  • use-after-free in DU
  • Add/remove logs, as appropriate and justified
  • Remove a lot of superfluous code in the RRC and simplify it (this MR adds new functionality, but reduces overall amount of code...)
  • Fixes to enable const-correctness
  • Rework the handling of DRBs at RRC: this was quite complex: first the RRC "guessed" a DRB ID when setting up DRBs via E1AP, and later chose one for real in fill_DRB_Configlist(). I remove that function, and instead allocate the DRB using generateDRB() before sending the message via E1AP. The rest of the logic is more or less the same.
  • correctly free the SDAP entity of a UE at gNB
  • improve the periodically shown MAC stats