sink: Make early drain reporting propagate down to filters

sink: Make early drain reporting propagate down to filters

The drain reporting improvements that were added to alsa-sink were only
being applied to directly connected sink inputs. This patch makes the
same logic also recurse down the filter hierarchy, so drains are
acknowledged more accurately (and not late) even if there is a filter
sink in between.

Also does some minor reorganisation of the code and sprinkles in some
comments as documentation.