Add support for Redis Sentinel

Related to #163 (closed).

The registry currently uses gomodule/redigo as the Redis client library. Contrary to go-redis/redis, this client has no built-in support for Sentinel. Alternatively, gomodule/redigo can be extended with FZambia/sentinel for Sentinel support. This is the method recommended by redigo.

Although I would prefer to switch over to go-redis/redis as it's more active and feature-rich, it does not support some of the configuration settings we currently expose (namely redis.pool.maxidle and redis.pool.maxactive), which means we would have to introduce a breaking change. To avoid this, for now we're moving forward with the least impacting change, which is using FZambia/sentinel with gomodule/redigo.