Use F1 internally: UE Context Release

Follow-up after !2070 (merged). It is not complete, and I am looking for feedback

• basic F1 release procedure
  • NAS release
  • UE UL failure without coming back
  • What is missing?
• Make it work over F1: minor changes needed
• Removed all RRC timers: moved to MAC, are we missing something? currently, if the CU instructed to release the DU, the DU forwards the RRC release and waits for UL failure to release the UE