Found with modernize-pass-by-value
Signed-off-by: Rosen Penev <rosenp@gmail.com>

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

On some VBR files, the single-step interpolation was very inaccurate
and inacceptable.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/720

No output at all is fine.  If the partition needs an output, it must
be moved there, but having just a null output isn't helpful.

By eliminating GetAnyClient(), we can have instances with no outputs
at all.

Make idle events per-partition, but leave Instance::EmitIdle() and its
underlying idle_monitor which broadcasts idle events to all
partitions.

For efficient traversal within one partition, e.g. for distributing
partition-local idle events.

These are global events.