Auto-recover from soft errors.

Allow the plugin to allocate the GetSong() return value.

Fixes wrong object URIs with duplicate base names.

Optimize the ProxyDatabase by invoking "stats" on the peer, instead of
visiting all songs.

Optimize the ProxyDatabase by invoking "list" on the peer, instead of
visiting all songs.

Let the C++ compiler take care for freeing the objects safely.

Fixes assertion failures in the Directory library.

Implementation incomplete, but sort-of-works.  DumpDatabase works, but
MPD is still hard-coded on the "simple" plugin.