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

A first use case for our libchromaprint integration added by commit
30e22b75

Allows removing a CommandResult::CLOSE check.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/234

Closes https://github.com/MusicPlayerDaemon/MPD/issues/516

Disable the 0.11 compatibility mode if the only argument is a filter
expression.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/506

implement for the "load" command the same logic used for the "add"
command: local clients can load playlist specified as absolute paths.

For relative paths the old logic is preserved: first look for a stored
playlist, then look in the music directory.

Must lock the mutex before calling any of the unprotected InputStream methods.

Closes #462