F1AP IDs: Add update, fix concurrency bug
I suspect a concurrency bug: For instance, during reestablishment, the
CU needs to update the DU UE ID under which the UE is identified in the
DU. Previously, the CU would remove, then add the DU UE ID info. At the
same time, the PDCP thread might look up the information. This can lead
to asserts. Modify the code to do the update under (one) lock.
Also, refactor some code.