RLC AM:

    - status reporting (38.322 5.3.4): implement this (not done because not
      clearly understood):

        "delay triggering the STATUS report until x < RX_Highest_Status
         or x >= RX_Next + AM_Window_Size."

    - send indication of successful delivery as soon as possible. Today we
      signal successful delivery in order. Probably not a big issue though.
