improve x2/rrc for ENDC

This MR introduces several enhancements in X2AP and RRC to deal with ENDC. The main goal is to deal with UE's on/off and with various errors.

The following work is included:

- DC prep and DC overall X2 timers
- X2 messages for UE release and DC timers' timeout

Open problems:

- what to do when the UE sends an scg Failure? This commit removes
  the UE. And this removal may not be correctly done
- UE removal in the gNB's PHY layer (something is done,
  but it is hackish and will need a rework)