Implement a confirm block callback

This patch adds a callback called when we receive feedback from the
client.
Also specify the timestamp when sending samples

This patch complements the SendSample() callback to take the timestamp
as extra argument.
Fix packet decoding logic

This patch totally reworks the logic of the packet decoding for the
server-side sound channel.
Merge pull request #1915 from hardening/server_side_sound_improvements

Server side sound improvements