Improved management for ZERO in platform-interval-secs

[IanMayo]

In PlatformTypes we have a column for default_data_interval_secs.

When we collate data for the timeline, we treat a gap of x times the interval as a break in coverage.

For legacy platform-types the field had a value of zero for almost all platform-types. This meant that the SQL interpreted every data value as a new mini-segment - so the dashboard_stats dataset was 700Kb and it took a long time to render/update. I set all the values to 60 seconds, and the dataset dropped to 23Kb, the update only took a second or so, and the serials got shaded in properly.

This issue probably won't arise again on client data. But it may save the day if we prevent it happening in the future. We can do this with some mangling of the data. In the SQL for dashboard_metadata we could append INTERVAL MISSING to any platform of a platform-type with zero default_data_interval_secs. We can modify dashboard_statistics to not process platforms of a platform-type with zero for the above field.