If the base class is not accessible, the "catching" the base class
won't work.  This caused the fatal error:

 terminate called after throwing an instance of 'LibmpdclientError'

Implementations shall use exceptions instead.

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.