Return false on error, not nullptr.

New in clang 3.6.

Fixes -Winconsistent-missing-override (clang 3.6).

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.

The new class is pluggable, to prepare for gzipped database files.

For now, the TextFile class remains, and will be refactored away
later.

Fixes potential crash.

Check for 0 in "device" and "inode" instead.

Remove the C list_head library and use type-safe C++ instead.

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

A SimpleDatabase instance can now "mount" other Database instances at
certain locations.  This is used to use a new SimpleDatabase instance
for each storage mount (issued with the "mount" protocol command).
Each such instance has its own database file, stored in the directory
that is specified with the "cache_directory" option.

Move to SimpleDatabase::GetSong() to give that method more control.

Code can now be reused in LookupSong().

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