Add daemon

Graphite-sender needs to report metrics regularly, as otherwise it will report null and maintenance flags will disappear if the viewed period is shorter than expected.

This PR fixes this by introducing a daemon method. Typically the last entry in the JSON should be null (it is the current minute and might not be written), but after aggregation a historic value should be stored. The daemon, running every minute, checks and modifies the previous minute if null by using the value from the antepenultimate minute. If this is also null, we send the value of NOMAINTENANCE, rather than persisting with null values indefinitely - which indicates that the daemon is not running).

The expected behaviour is that the following series: [96, 0], [null, 60], [null, 120]

becomes [96, 0], [96, 60], [null, 120]