nr pdcp: rework reestablishment
Security part needs to be checked, may fail to work. But if it fails, it was probably not working before either.
openair2/RRC/NR/rrc_gNB.c probably needs some more work, it is a bit ugly to reestablish all the drbs without checking if they exit. It will generate warnings at runtime. I didn't do it because I don't know the proper way.