[LMP] Don't recreate ~/.leechcraft/lmp/covers if it's not used.

For #1819.
[LMP] Complain to logs about AA cache path creation failure.
[LMP] Avoid deadlocks on on-start collection creation.
[XSD] Support CACHEDIR in paths for #1819.
[LMP] Store AA cache in XDG cache by default for #1819.
[LMP] Fallback AA cache path is in XDG cache now.

This finally closes #1819.