x2: first implementation of ID management, some cleanup
Some deadcode was removed. The implementation of the ID management is not finished. It will only work if the handover is successful. No error recovery has been done. For error recovery, we need to implement the X2AP timers. As it is, in case of error, an ID might very well never be released. It has been tested with 1 and 2 UEs doing handover more or less at the same time.