This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

playlist_domain is no longer used for class Error.

Necessary to preserve type information.  The try/catch sequence didn't
work previously.

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.

Avoid GLib.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.