echo-cancel: Limit the maximum sink/source latency

echo-cancel: Limit the maximum sink/source latency

On systems with constrained CPUs, we might run into a situation where
the master source/sink is configured to have too high a latency.

On the source side, this would cause us to wake up with a large chunk of
data to process, which might cause us to exhust our RT limit and thus be
killed.

So it makes sense to limit the overall latency that we request from the
source (and correspondingly, the sink, so we don't starve for playback
data on the source side).

The 10 blocks maximum is somewhat arbitrary (I'm assuming the system has
enough headroom to process 10 chunks through the canceller without
getting close to the RT limit). This might make sense to make tunable in
the future.