core, alsa: Better drain reporting

core, alsa: Better drain reporting

Previously, a drain request was acknowledged up to two hw buffers
too late, causing unnecessary delays.

This implements a new chain of events called process_underrun
which triggers exactly when the sink input has finished playing,
so the drain can be acknowledged quicker.

It could later be improved to give better underrun reporting to
clients too.

Tested-by: Dmitri Paduchikh <dpaduchikh@gmail.com>
Signed-off-by: David Henningsson <david.henningsson@canonical.com>