Switch from our custom datastore cache(s) to ndb's with custom policies?

[snarfed]

Would be nice to ditch our own code for this! https://cloud.google.com/appengine/docs/legacy/standard/python/ndb/cache , https://googleapis.dev/python/python-ndb/latest/context.html#google.cloud.ndb.context.Context.set_cache_policy

It has a bug/weird behavior that we'd probably need to work around though, https://github.com/snarfed/bridgy-fed/issues/558 . More: https://github.com/snarfed/bridgy-fed/issues/558#issuecomment-1603253691