This patch changes how `Feature`s configs work - they are now required
to use a separate config object (each `Feature` their own class, derived
from `FeatureConfig`) that is to be used as a `perconfigobject` storage.
`Feature`s themselves are not supposed to interact with their config
objects directly, instead using JSON-serializable type, which should
allow us a universal way to change `Feature`'s config variables during
execution.
Anton Tarasenko