Effect leak fix

This fixes the leaks we experience when changing effects and effect chains. 
This is somehow a band aid because now the EngineEffect is deleted inside the audio callback 
and all EnginEffect used at Mixxx shutdown are leaked together with some other stuff. 

IMHO we should merge it anyway, because a possible buffer underflow during effect change is not that critical as a out of memory crash after 2 h party.

The other issues can be fixed in a separate PR
 
The reason you want an EffectPointer here is that it guarantees that m_pEffectParameter is not deleted while it is loaded.