With the "wave" encoder, this has no effect, but it's more correct.

This uncomments the code which had been present already in the first
Snapcast commit (copied from the "httpd" output plugin), but I
commented it because I did not know whether I needed to send silence
samples to all Snapcast clients.

As a side effect, this fixes playback when no Snapcast client is
connected; this was broken because Pause() always returned a positive
value when there were no clients.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1310

emplace already calls std::pair. No need for it again.

No need to emplace when calling std::make_shared.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with readability-use-anyofallof
Signed-off-by: Rosen Penev <rosenp@gmail.com>

New experimental code, first draft - it works, but there's a lot left
to do.  Just look at all the TODO comments.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/975