Optimize processor settings storage

We currently store all the settings for all processors, even the disabled ones, and because of the "Fields to run on" option and its current storage, this can become rather lengthy, making the YAML file more or less unreadable. So, a few suggestions:

• Almost definitely: Fix the FieldsProcessorPluginBase class to only store the enabled fields, not also [KEY] => 0 mappings for the disabled ones. Also, just storing a numeric array for the enabled fields and transforming that to an associative array upon processor construction should be sufficient and would also make this look a lot nicer.
• Then, do we need the settings of disabled processors? In 90% of the time, they will just be the defaults, and in most other cases it won't matter much if we just discard the changes. If we do think that we should keep user changes here, maybe just comparing the submitted values to the defaults and only saving those values that got changed would make sense? (This could even be also done for the enabled processors, if we are fairly sure the defaults won't change normally.)