Enforce storage/retrieval conversion to UTC

[bpkroth]

Further improvements on microsoft#716.

Sqlite does not store timezone info even if provided, so retrieving the data may be interpretted according to the host machine's local timezone, which may or may not be UTC.

To mitigate this, this change ensures that all timestamps are

1. first converted to UTC before storing into the DB,
2. converted (or augmented with zoneinfo) to UTC on retrieval

Additionally, we expand the tests to check for this behavior, first with some additional conversion matrixes when telemetry or status data is received in implicit local vs. explicit timezone data as well as executions where the implicit local timezone has be overridden with a TZ environment variable, to simulate different default timezone hosts.

[bpkroth]

Oops. Accidentally pushed this to your branch. Rolled it back for review first and redid in #13