Speed up ATF by using the entries in the database whenever possible instead of always re-reading the metadata.

Note: I have yet to verify that this code works as intended. I'll test ASAP, but I believe it should be correct. Hopefully I've
introduced no new bugs... :-)

Martin -- can you give it a go with your 6300-song playlist and let me know how much of a speedup (if any) is gained with this code?
Thanks!