States: set progress floating point directly

Right now fetchers and parsers are supposed to set total and progress, later a progress floating point number between 0.0 and 1.0 is computed on the FeedsSource level.

This means the assumption is that a natural number total and progress are managed at $state->total and $state->progress by plugins.

We should consolidate this into a single float 'progress' that is set by plugins with a helper function. Thus we consolidate progress tracking into a single floating point.