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']))))

because an UE might be associated with multiple cells of one eNB
due to e.g. Carrier Aggregation.

Now, after we did preparatory amari.drb refactoring and taught BitSync
to be multicell-aware, we can finally remove the limitation and correct
the Sampler to handle UEs associated with multiple cells at the same
time.

The handling is mostly straightforward: for every frame the Sampler needs
to estimate time of active transmission and amount of transmitted bytes.
The transmitted amount is Σcell(tx_bytes) and we estimate transmission
time on different cells C₁ and C₂ as

    tx_time ∈ [max(t₁,t₂), min(t₁+t₂, δt/tti)]

In other words when transmission to/from UE on C₁ and C₂ is going in
parallel in one frame, the time it takes to transmit current part of UEs
data

    1) cannot be less than time spent on each cell
       (equals to the maximum if transmission on Ci was fully performed
        in the same subframes where transmission on Cj was active)

    2) cannot be more than the sum of time spent transmitting on each cell
       (equals to the sum if transmission on C₁ and C₂ was performed in
        distinct subframes of the frame), and

    3) cannot be more than the whole frame
       (t₁+t₂ could become more than that if there is some overlap in
        subframes where C₁ and C₂ transmissions were done)

The patch implements this logic when updating data flows of an UE.

Amari.drb now works for multicell configurations.

There are one-cell unit tests for Sampler already added in 2a016d48 and
now I manually verified that `xamari xlog x.drb_stats` works ok with
multicell eNB configurations.

Unittests for Sampler in multicell environment are left as TODO.