Use this function instead of CheckError() when we already know an
error has occurred.

The old struct config_param remains only for top-level string options.

The Release Track Id uniquely identifies a recording on a release - that
is, even if a recording appears twice on a release (meaning that the
combination of recording and release id are not enough to figure out
which one it is), the release track id will allow differentiating the two.

The tag names are taken from
https://musicbrainz.org/doc/MusicBrainz_Picard/Tags/Mapping

Workaround for assertion failure in PrintUniqueTag().

Use milliseconds precision to reduce rounding errors.

Fixes potential crash.

Fixes crash bug because mpd_connection_get_async() was called without
a connection.

Ignore the storage configuration if FLAG_REQUIRE_STORAGE is not set in
the DatabasePlugin.

The new method IsPlugin() replaces the "is_simple" flag.

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

Check mpd_response_finish() before using mpd_song.  Don't skip this
check even if the mpd_song is non-nullptr.

Reduces overhead because we need to manage only one memory allocation.
According to valgrind/massif, we save 7%.