hotfix: better management of s-tmsi reconnect
The code was changing the rnti in the red-black tree without removing/reinserting the context. This is wrong because the rnti is a key of the red-black tree and you cannot change the key in a stored item at will. There are also modifications of the function mac_eNB_rrc_ul_failure which has to remove the UE in any case. And rrc_mac_remove_ue has been changed for the case where the UE is not found.