This patch provides next building block for E-UTRAN IP Throughput KPI
and continues

    d102ffaa (drb: Start of the package)
    5bf7dc1c (amari.{drb,xlog}: Provide aggregated DRB statistics in the form of synthetic x.drb_stats message)

Quoting those patches

    The scheme to compute E-UTRAN IP Throughput is thus as follows: poll eNB at
    100Hz frequency for `ue_get[stats]` and retrieve information about per-UE/QCI
    streams and the number of transport blocks dl/ul-ed to the UE in question
    during that 10ms frame. Estimate `tx_time` taking into account
    the number of transmitted transport blocks. And estimate whether eNB is congested or
    not based on `dl_use_avg`/`ul_use_avg` taken from `stats`. For the latter we
    also need to poll for `stats` at 100Hz frequency and synchronize
    `ue_get[stats]` and `stats` requests in time so that they both cover the same
    time interval of particular frame.

    Then organize the polling process to provide aggregated statistics in the form of
    new `x.drb_stats` message, and teach `xamari xlog` to save that messages to
    `enb.xlog` together with `stats`.

    Then further adjust `amari.kpi.LogMeasure`						<-- NOTE
    and generic `kpi.Measurement` and `kpi.Calc` to handle DRB-related data.

So here we implement the noted step:

We teach LogMeasure to take x.drb_stats messages into account and update IP
Throughput related fields in appropriate Measurement from x.drb_stats
data.

This process is relatively straightforward besides one place: for stable
output E-UTRAN IP Throughput is required to be computed without taking
into account last TTI of every sample. We don't have that level of
details since all we have is total amount of transmitted bytes in a
burst and estimation of how long in time the burst is. Thus we can only
provide an estimation for the E-UTRAN IP Throughput as follows:

    DRB.IPVol and DRB.IPTime are collected to compute throughput.

    thp = ΣB*/ΣT*  where B* is tx'ed bytes in the sample without taking last tti into account
                   and   T* is time of tx also without taking that sample's tail tti.

    we only know ΣB (whole amount of tx), ΣT and ΣT* with some error.

    -> thp can be estimated to be inside the following interval:

             ΣB            ΣB
            ───── ≤ thp ≤ ─────           (1)
            ΣT_hi         ΣT*_lo

    the upper layer in xlte.kpi will use the following formula for
    final throughput calculation:

                  DRB.IPVol
            thp = ──────────              (2)
                  DRB.IPTime

    -> set DRB.IPTime and its error to mean and δ of ΣT_hi and ΣT*_lo
    so that (2) becomes (1).

for this to work we also need to introduce new fields to Measurement
that represent error of DRB.IPTime. The hope is that introduction is
temporary and should be removed once we rework DRB stats to provide B*
and T* directly.