Refactor backend plugin initialization
Refactor backend plugin initialization

The plugin initialization routine, performed in StatsProducerBase.__init__(),
had been split into two loops over the plugin classes to allow some plugin
classes to create their model classes dynamically in make_model(), which was
convenient but technically unnecessary. That method has been done away with,
allowing those two loops to be merged back together.

yazman/statscache,yazman/statscache,yazman/statscache