This commit implements reestablishment during handover when occurring at
the source DU, either as a handover Failure or "normal" reestablishment
(something else went wrong). In this case, the CU will trigger HO
cancel, which in F1, means to cancel on the target DU. Also, undo any
previous ID changes. Retain the old CellGroupConfig (before handover)
for the case of reestablishment at source. If not needed, it is freed
after the reconfiguration complete.

Reestablishment on target cell during handover cannot be handled by the
gNB (yet). It is the same as with a normal reestablishment (not during
handover) on a different cell: that cell has no notion of the old UE, so
we cannot simply indicate the old UE to reestablish; at least, we would
need to cleanup the CellGroupConfig, and I am not sure this works out of
the box, either.