Updated the plugin to use the NCount and Nth operator

The two operators allow for optimized non numeric and waveform data loading. Depending on the number of points in the archive, only every n-th point will be fetched, so one ends up with a reasonable number of data points in the selected time range.

The code should be backward compatible with the previous versions of archiver appliance (which didn't have these 2 operators yet), but there will be some exception stack traces printed to console.

The latest version of the appliance (12th April) has a bug in these two operators, which makes them unusable. Therefore, if you are using the 12th April version, do not update this plugin. After Murali commits my patch, things should work OK.