Incorrect caching of feed targets

In _entityreference_feeds_get_targets an internal cache exists, keyed with target_type and target_bundle. The reasoning behind this is that the targets are unique per entity_type and bundle, and could be shared across fields. This is not actually the case since the field_name is baked into the key, name, description etc though, so this will break if there are more than one entityreference field referencing the same entity type and bundle. We could perhaps break out the reusable information, cache this, and just apply the field specific information for each target.