Cache Policy Upgrades

[cicdw]

This PR updates cache policies in the following ways:

• updates the signature on compute_key to be closer to the original cache_key_fn signature
• auto-converts cache_key_fn's to CacheKeyFnPolicy's for backwards compatibility
• adds an overwrite flag on transactions that can be used to ignore existing records; the engine currently matches this to the original refresh_cache functionality

NOTE: cache expirations are not currently wired up; that will require some extensions to cache policies that will need to be handled separately

cc: @EmilRex (I won't keep doing this I promise haha - this just dovetails nicely with our conversations this week so wanted you to know when these milestones are complete)

[jlowin]

Did you know if you at-@EmilRex three times in three PRs he has to grant you three wishes? I don't make the rules

[chrisguidry]

I love it when a plan comes together, so nice to unify the old cache functions under the new scheme

[EmilRex]

Did you know if you at-@EmilRex three times in three PRs he has to grant you three wishes? I don't make the rules

It's true! I'm always down for a challenge.