The mechanism to extract a timestamp from JSON needs generalising

[amrc-benmorrow]

Currently when an Edge Agent connection returns JSON, the Edge Agent will automatically (and unconfigurably) look for a timestamp property and treat this as the metric timestamp. This is obviously incorrect and unsafe; there is no way of knowing the timestamp property is supposed to be interpreted as a metric timestamp.

This is a useful feature but needs to be configurable. This means:

• A 'timestamp path' alongside the existing 'path' configuration for each metric. I don't think we want to support timestamps read from a different address.
• A 'timestamp format' giving options of at least 'Unix seconds' and 'Unix milliseconds'. We should also allow other sensible things, like ISO date strings and maybe other epochs.

The timestamps parsed from the data MUST be sanity checked. Publishing data way in the past or in the future as a result of a mis-parsed timestamp will play havok with the database.