For querying tags, the real song URI should be used, because if the
(display) URI is different, requesting it will not produce a usable
response.  This is a theoretical problem because none of the existing
playlist plugins sets the real_uri.

This requires changing the URI comparison in playlist::TagModified().

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1154

The former was deprecated with C++14.

According to the C++11 and C++17 standards, both files are identical.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

This commit also puts an instance of RemoteTagScanner into the
Instance class, and hooks it into the "add" and "addid" commands.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Fix for the problem where order with priorities gets out of whack in case it's
reordered by SetRandom() while another song is currently playing.
What happens is, if some song is already playing and you have set some
priorities before switching on the random mode, and then turn the mode on, the
original code swaps position of the first song in the order (i.e., the highest
priority song) with current, so that current is 0 (which it should be). The
problem is, the "original" first song then goes to the place "current" song was
after reordering, wherever that is, instead of going after the "current" song.
This patch fixes the issue.
Also the fix makes MoveOrder() public, because why shouldn't it be, anyway.  It
certainly makes more sense than just having SwapOrders() public for some
reason.
Signed-off-by: Eugene Baklanov <miltenfiremage@gmail.com>

Fixes disappearing duration of remote songs during playback.

See http://bugs.musicpd.org/view.php?id=4492

A priority should be a volatile thing: it should schedule a song for
playing back once, but it should not affect the next time the queue
gets played.

Empty currently, but code will be added song.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.