Changes

• completed last-mile latency measurement: scamper trace version

  Adds both task lml (executable netrics-lml) and task alias
  lml-scamper (netrics-lml-scamper) for clarity.

  This version of the measurement is added to the default configuration
  but commented out as it may not be enabled by default.

  For speed and niceness, this lml test will randomly select an endpoint
  to trace, from a default set of Google DNS as well as CloudFlare DNS.
  (The former is generally speedier to respond; regardless, it might be
  "nice" to round-robin, and for that matter to fall back in the event of
  response failure.)

  Otherwise:

  Measurements' schedules in default configuration now use hashed cron
  expressions to discourage their collision (though any which may not
  run concurrently will be configured as such).

  completes initial draft of last mile latency test #13

  part of built-in measurements #3

• extended measurement task utilities to simplify/DRY tasks

  in use now by ping latency measurement:

  • basic input parameter schema for global and individual configuration
    of measurements' result outputs -- via netrics.task.schema.extend

  • helpers for measurement tasks' individual schema definitions:
    netrics.task.schema

  • decorator for retrieval of tasks' input parameters:
    netrics.task.param.require

  • software-default set of ping connectivity/latency targets:
    netrics.measurement.common.default

  for future use by other measurements:

  • internet connectivity-checking decorator:
    netrics.measurement.common.require_net

  • parallelized domain name resolution helper:
    netrics.measurement.common.AddressLookups

Replaces #13

Part of #25