Make elan read from redis (and a bunch of other improvements to redis).

[fische]

This PR makes elan read from Redis before hitting the bucket, so we can save on bucket operations. It does not make it write to redis as we were worried it would highly increase the number of evicted keys. We might experiment with that in the future.

There are a bunch of additional improvements to the redis config as well:

• to avoid maintaining 2 separate sets of flags, I've moved the redis options from the mettle worker package to //redis;
• to avoid reimplementing the error rate limiter we use in the worker code, I implemented the Limiter interface from the redis package, so it is now supported natively within the clients;
• I've also made the redis MaxSize configurable, as we might want to tweak it at some point to make sure we use Redis to its full potential.

Those additional changes came in as I faced the issues they are trying to address. They are completely separate from the elan change and could be moved to a separate PR. If you think this PR is too big, I'm happy to split it.