Set 1m idle timeout on Redis connections

[sethvargo]

Centralize our Redis configuration and set a 1m idle timeout on all Redis connections

Release Note

Set a 1m idle timeout on all Redis connections. `REDIS_HOST` and `REDIS_PORT` are now prefixed based on their scope (e.g. `CACHE_REDIS_HOST` and `RATE_LIMIT_REDIS_HOST`). This enables using a different Redis cluster or configuration for rate limiting vs caching.

/assign @mikehelmick
/assign @icco

[sethvargo]

This needs to be it's own package because otherwise we have a circular dep

[google-oss-robot]

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This pull-request has been approved by: mikehelmick, sethvargo

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