protocol-native: Lower default minreq in low-latency scenarios
If minreq is not explicitly specified, it was always initialized to
20 ms (DEFAULT_PROCESS_MSEC). However when the total latency is not
much higher than 20 ms, this is way too high. Instead use
tlength/4 as a measure: this will give a decent sink_usec in all
modes (both traditional, adjust latency and early request modes).
This greatly improves PulseAudio's ability to ask for data in time
in low-latency scenarios.
Signed-off-by: David Henningsson <david.henningsson@canonical.com>