Make samplerate an explicitly-passed variable in the effects framework.

Per @rryan 's suggestion to remove control-object dependencies from the effects framework (https://github.com/mixxxdj/mixxx/pull/316/files#r16438349).  It didn't make sense to set samplerate in the initializers, though.

This avoids make the effects framework depend on control objects and prevents
possible sample-rate changes during effect processing.

TESTED=existing tests pass, and sanity-checked effects and sample rate changes.
Is this one still in use? If yes, it would be nice to get rid of it, to encourage new software to use the one passed with process()