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.

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

Behave like STL.

The new Response instance in the `catch` block didn't have the
`command` attribute set, so the error response didn't indicate which
command had failed, which however is required in the MPD protocol.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/628

Clarify that those can't throw, preparing for the next commit.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/42

Use the same chunk size for all binary commands.

Move code from read_stream_art().

Instead of passing tag and group, pass an array of tags.  To support a
nested return value, return a nested std::map of std::maps.  Each key
specifies the tag value, and each value may be another nesting level.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/408

Eliminates a number of allocations, because callers don't need to copy
the strings to a newly allocated buffer only to null-terminate them.
And most callers don't need to have a null-terminated string.

GetChromaprintCommand::DecodeStream() is called without holding the mutex.

Just like std::condition_variable, which however has no way to specify
the std::mutex directly.

Commit b3a45833 added a LocateUri()
call to several playlist commands, which applied InputPlugin URI
scheme verification to playlist URIs.  This broke the SoundCloud
playlist plugin which uses "soundcloud://" URIs for which no input
plugin exists.

This commit allows the caller to specify the kind of plugin which
shall be used to verify the URI.  Right now, only "input" is
implemented; "storage" uses the "input" verification for now; and
"playlist" has no verification at all (for now).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/528