Since 2a016d48 (Draft support for E-UTRAN IP Throughput KPI) there was a
hardcoded limitation that x.drb_stats generation works with 1-cell
configurations only. However we do use multicell eNB configurations and
on such configurations `xamari xlog x.drb_stats` was failing on eNB
side with

    raise RuntimeError(("ue #%s belongs to %d cells;  "+
        "but only single-cell configurations are supported") % (ue_id, len(ju(['cells']))))

Start fixing that.

As the first preparatory step to support multiple cells with x.drb_stats
reorganize amari/drb.py code a bit: _QCI_Flow works at whole QCI
transmission level which can aggregate and cover multiple cells, but
xl_use_avg and rank are per cell things and they can be different for
different cells.

-> Start preparing to handle them in per-cell way by adjusting the code
   to take those values into account at higher level in computation stack
   where we still have cell context.

No support for multicell yet - only preparatory non-functional changes
currently.