Changes

StatsD_Checker: Allow running without VTY support

New features are added to the public API of StatsD_Checker which make it
possible to use it...

* without "stats reset":
Feature to take snapshots (f_statsd_snapshot()) which can later be
used to validate expectancies with values relative to the snapshot,
using API f_statsd_expect_from_snapshot().
This way, one can do:
"""
var StatsDExpects statsd_exp := { /* relative expectancies here... */ };
var StatsDMetrics statsd_snapshot := f_statsd_snapshot(f_statsd_keys_from_expect(statsd_exp));
/* do some test stuff here changing the state of the IUT... */
f_statsd_expect_from_snapshot(statsd_exp, snapshot := statsd_snapshot);
"""

* without polling ("stats report"), aka with periodict reporting:
New parameter wait_converge in f_statsd_expect(), which allows
overcoming race conditions with StatsD server processing older incoming
metrics due to periodic reporting.
This feature also allows a test to wait until a state changes in the
IUT.

Change-Id: Ie1180a5b674504864309c3b9b11bfcf5256d9178