Add complete support for Redis sentinels in Async mode

Support for sentinels in async mode have been added upstream: https://github.com/3scale/apisonator/pull/362.

However, it's still not possible to connect to password protected sentinels, because of a bug in the async-redis gem. Check: https://github.com/socketry/async-redis/pull/48.

When that PR is accepted. Consider upgrading async-redis for apisonator, or write a monkey patch to fix the bug.

On the other hand, sentinels protected with TLS encryption aren't supported either. We should write a PR for async-redis to add it.