Fixed a data race between DL processing of RAR and UL scheduler, which
might result in dropping MSG3.
It was possible that the UL scheduler for the UL slot which RAR
indicates as MSG3 slot was already run by the time the RAR was decoded.
Therefore when DCI for RAR is decoded, assume worst case and halt the
earliest corresponding UL scheduler call until RAR is decoded solving
the data race. This also in turn halts all later scheduling due to
sequential nature of the UL scheduler calls.