Fix possible noises and optimization with dmix

Fix possible noises and optimization with dmix

dmix transfers data in asynchronously so that it doesn't write on the
last active period.  This will allow the arbitrary buffer size in
future, too.

The hwsync call to slave is removed from commit and hwsync callbacks.
This hack was added to fix the noisy output (typically on xmms/bmp)
but it doesn't happen any more.  Now the hwsync behavior is as same as
on 1.0.8 (i.e. hwsync is called only when slowptr option is set).