massive database-centric update

gw2api_items_database.pl is now build-aware: skips items already seen in
current build, wipes cache on new build, logs old version of item data.

API.pm: added retry functionality around cache set action (was getting
temp filename conflicts in threads), utility methods to clean expired or
all item/recipe from cache.

Item.pm: renamed a few things, added soft validation for enumerated
fields, updated derivation logic for complex fields. (Still need to
update pod)

SQL: New database structure