update boards/db to use new format for tree.store

callback(options) instead of callback(err, storedObject)
update threads/db to use new format for tree.store

callback(options) instead of callback(err, storedObject)
update users/db to use new format for tree.store

callback(options) instead of callback(err, storedObject)
boards/db parentKey -> parentKeys in tree.store()
boards/db tree.children -> tree.children(options)
threads/db parentKey -> parentKeys in tree.store()