Remove race possibilities from slip mode

I think this was causing https://bugs.launchpad.net/mixxx/+bug/1362394, and I know for sure there were races in this code, so even if it doesn't fix that bug it's an improvement.
Ah, I beginning to understand. I think it is required to have a additional slip status, only processed by audio callback and to ditch the toggle flag: 
```
if(m_bSlipEnabled != m_bSlipEnabledProcess) {
    bool toggle = true; 
    bool enabled = m_bSlipEnabled; 
    m_bSlipEnabledProcess = enable: 
}
```
use m_bSlipEnabledProcess in hintReader()