Previously, whenever the output device sample time changed from
'invalid' to 'valid', the AudioBuffer fifo in the AudioIODeviceCombiner
was cleared. This caused a data race, since the clear operation was not
mutually exclusive with writes from the input device.

This change causes the AudioIODeviceCombiner to keep track of the
timestamp of the first input device callback after the output device is
invalidated. The output device is unable to read from the fifo until its
timestamp exceeds the stored input device callback timestamp.