Improvements in handling UE context in gNB DU
The goal of this MR is to improve the handling of UE context in gNB DU.
To achieve this goal, the UE structure is created as soon as the gNB
receive a PRACH indication. At that stage the UE is placed in a new
structure containing UEs performing RA, untill RA is complete where it
is moved to the list of connected UEs. To simplify the code and to
improve the handling of initial procedures (see #751), DCI 10 and DCI 00
is used at every stage until the DU receives from CU the indication of
UE RRC reconfiguration being completed.