cmApBufUpdate() sets an input/output timestamp on each device according to the audio packet timestamp.
cmApBufGetIO() returns the timestamp associated with each returned buffer.
Added cmApBufOnPortEnable() to zero the timestamp when a port is enabled.
This implementation of the audio timestamping processes is a hack because
only the first timestamp is stored (after a call to cmApBufOnPortEnable())
and all subsequent timestamps are generated by incrementing this value according
to the incoming/outgoing sample count.
