This patch provides next building block for E-UTRAN IP Throughput KPI
and continues d102ffaa (drb: Start of the package). Quoting that patch

    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	<-- NOTE
    new `x.drb_stats` message, and teach `xamari xlog` to save that messages to		<-- NOTE
    `enb.xlog` together with `stats`.							<-- NOTE

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

So here we implement the noted step:

- add drv._x_stats_srv server that polls eNB at 100Hz rate, uses Sampler
  to extract bursts and aggregates information about those bursts.

- teach xlog to organize servers for synthetic messages and communicate
  with them, and register drv._x_stats_srv as such server to handle
  generation of x.drb_stats message.