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.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

Works around gcc 4.6 bug.

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.