Move cache to sqlite

[GoogleCodeExporter]

The current MusicCollection implementation is very slick, but very memory
intensive.  People with very large collections would have to set a very
large heap size to be able to use Harmonium at all.

I wonder if we could store less when parsing the cache at startup, and look
things up from the cache on-the-fly in more situations, so we don't have to
hold the whole collection in memory.

This would be a pretty major change, and decidedly not trivial.  It might
make sense to get an estimate how much heap memory is used per song in the
collection, so we could draw a correlation between library size and heap
size.  With this information we could better decide if it's worth taking
this on.

Storing the cache in something like sqlite might be worth exploring to help
with this and generally make the cache faster.

Original issue reported on code.google.com by [email protected] on 24 Dec 2009 at 3:30