upload_static_files_to_s3: Don't list the whole bucket to find changes.

The static files S3 bucket has been getting a lot larger recently,
between subreddit stylesheets being in there and the static file cleaner
being disabled due to a bug. This is causing the deploy process to take
upwards of 3 minutes just to determine that no files need to be uploaded
to the bucket.

As a short-term workaround, this changes the uploader to check each key
individually with an S3 HEAD request rather than listing the whole
bucket. This is slower than best case of listing the bucket, but is
significantly faster than the current condition (~25 second runtime
now).
Add new dedicated memoize cache pool and start dual-writing.

Memoize creates very large objects that take up lots of space
and has weird behaviour when it gets evicted. We get some isolation
and monitoring by moving it to its own pool. Since it is sensitive
to split-brain stuff, we start out here by dual-writing to the new
pool.
memoize: Start reading from new memoize cache pool.
memoize: Stop dual-writing to old cache pool.