SI-8316 fixes resetAllAttrs abstinence syndrome

Apparently our addiction to resetAllAttrs was much more sneaky than we
thought previously. Even though everything was mostly going as usual,
whitebox macros expanding into reified trees that then expanded and reified
again created symbols that failed to be removed by resetLocalAttrs.
The cleanest solution here would be to adopt scala/async's approach,
but first we need to plug the leak to fix Slick before RC1 goes live.