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%.

Send "idle" to the other MPD whenever there's nothing else to do and
forward incoming "idle database" events to all our MPD clients.

Allow database plugins to announce that they have been modified.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

Refactor SimpleDatabase::GetLastModified() to be generic for all
plugins.  Remove the SimpleDatabase assumption from db_stats_print(),
allowing it to be implemented by all database plugins.