nr: improve x2/rrc for ENDC

This commit 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 (a next commit is coming,
      but it is hackish and will need a rework)

executables/nr-gnb.c

@@ -139,7 +139,6 @@ extern void add_subframe(uint16_t *frameP, uint16_t *subframeP, int offset);
 //#define TICK_TO_US(ts) (ts.diff)
 #define TICK_TO_US(ts) (ts.trials==0?0:ts.diff/ts.trials)
 
-
 static inline int rxtx(PHY_VARS_gNB *gNB, int frame_rx, int slot_rx, int frame_tx, int slot_tx, char *thread_name) {
 
   sl_ahead = sf_ahead*gNB->frame_parms.slots_per_subframe;