This implementation might work, but it simply triggers RRC setup on the new
DU when we might send a RRC reestablishment speeding up the whole
connection setup. For this:

- we should send the F1 UE context setup to the new DU
- we should still do the reestablishment, containing the reestablishment message

The latter should be sent once we received the F1 UE context setup
response. We don't need a new context. send a release to the old DU (and
don't release our own context!!).

We might also send the reestablishment to the UE encapsulated with the
F1 UE context setup response. Need to investigate if this is feasible
(because if the DU rejects [can that happen here?], the UE will still
send reestablishment complete -- what then?).

Also, we would need to send the E1 bearer context modification to the
CU-UP.