Add some documentation to SoundManager methods about concurrency issues.
Get rid of QHash for storing input buffers to delete and use a QList instead.
Remove request buffer mutex since there is only one clock reference device.
Move output buffer composition from SoundManager to SoundDevice.

This fixes a race condition in the use of the mono-downmix buffer. Multiple
soundcards could concurrently use the downmix buffer in SoundManager which leads
to data corruption. Making the downmix buffer owned by the SoundDevice prevents
this.