pulsesink: Update segdone periodically

pulsesink: Update segdone periodically

This makes sure that we update segdone based on the read index received
during latency updates. As the comment notes, we make some compromises
to deal with the fact that segdone is a segment multiple, while the read
index offers finer granularity. The updates are also not very often
(100ms since that is how often automatic timing updates are provided).

All this is required for the baseaudiosink sample alignment code to work
at all.

https://bugzilla.gnome.org/show_bug.cgi?id=694257