Refactor LC handling at MAC

This MR refactors some code at MAC to better add/release new LCs. Specifically

• use the seq_arr to handle LCs, and add functions to add/release
• sort LCs by priority. For SRBs, use priority from the spec. For DRBs, use the minimum priority of all QoS flows mapped to the DRB (or 100)
• Remove process_CellGroupConfig(), which remains empty after putting the RA setup code in the function that adds a UE in NSA