Replace usage of `Cow::as_ref()` when passing args to Redis

[caass]

In redis_store.rs, we need to get values out of Cows to pass along to redis via ToRedisArgs.

Currently, we do this with a kind of annoying call to key.as_str().as_ref() (example 1, example 2, example 3, example 4, and example 5).

Once redis-rs/redis-rs#1219 lands, we should be able to replace these with just key.as_str.

In the event I achieve my other goal of writing a type-safe wrapper around a redis connection that only allows for accessing with prefixed keys, then we should still do something about this but inside of that layer instead.