[CLEANUP] swap cleanups

Before cacheCtx was introduced, care had to be taken to revert any store changes made during msg handling in case of errors. In swap this involves maintaining a local pool cache, avoiding storing updates until we're sure there are no errors, and then reverting to the original pools if anything goes wrong.

This branch removes this cache and stores updates inline during the handling, reducing the extra complexity of maintaining a cache. It also moves some validation inline as well, placing the checks closer to the code which relies on them.

Broken out into separate commits in an effort to be more readable.