Omit a few methods that are not applicable.

This was bugged, because it did not scale the "size".

Lists files and directories.  Supports storage plugins.

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.

The problem with std::queue is that it doesn't give us enough control.
The method Clear() is a kludge already, but soon, we'll need
filtering.

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

Code can now be reused in LookupSong().

Helps avoid unnecessary archive scans during database update on a
fresh MPD process.

Fixes crash after database update.

Fixes initialization failure.

There is no advantage in using g_setenv().