The existing implementation has been utterly broken forever; I cannot
explain what it actually does, but it doesn't do what the
documentation says.

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

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.

Empty currently, but code will be added song.