echo-cancel: Try to minimise in-flight chunks in snapshot latency

echo-cancel: Try to minimise in-flight chunks in snapshot latency

We don't always know whether the in-flight memory chunks will be
rendered or skipped (if the source is not in RUNNING). This can cause us
to have an erroneous estimate of drift, particularly when the canceller
starts.

To avoid this, we explicitly flush out the send and receive sides of the
message queue of audio chunks going from the sink to the source before
trying to perform a resync.