After commit 1afa33c3, an old bug was revealed:
SimpleDatabase::GetSong() constructs an ExportedSong instance by
moving the return value of Song::Export(), which causes the
LightSong::tag field to be dangling on the moved-from
ExportedSong::tag_buffer.  This broke tags from CUE sheets.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1070

This enforces the "not nullptr" rule.

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

Reduce overhead.  LightSong can always point to the "real" allocated
string.

The UPnP database plugin can now show relative song URIs for remote
songs.

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.