[P3] Replace node-redis with ioredis

[willroberts]

We're currently using [email protected], which has poor support for connection timeouts.

Replacing this with [email protected] fixes the above, but requires some changes. For example, the zrevrankAsync function does not exist, so we need to use zrevrank instead and remove the .then(result) chain.

Alternatively, upgrading node-redis may help with these issues.

See https://github.com/luin/ioredis/wiki/Migrating-from-node_redis