Changes

StatsD_Checker: Allow building without VTY support

Some programs may support exporting to statsd, but may not support the
Osmocom VTY set of commands to send reports, or not have a VTY at all.

The current StatsD_Checker implementation as it is now is not totally
useful without VTY code. a Follow-up patch will provide new APIs and
improved implementation to use it without VTY.

Change-Id: I5421c76e4f303fd16d4db945a1c69910e40ac820

StatsD_Checker: Simplify by getting rid of StatsDExpectPriv

Get rid of extra record, we have plenty of them and we'll have more once
we add new APIs to work without VTY.
While doing so, simplify the expect loop by maintaining a counter.

Change-Id: Ib14c838c5f0647d050249853327671542182636f

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