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.
the cached m_sampleRate can be outdated. And writing it from an other thread is not thread save. 
Just use the get method when calling process().