Add support for sentinels in Async mode

As far as I know, Apisonator doesn't support Redis sentinels on async mode.

Evidences:

• We sent a patch to async-redis upstream: https://github.com/socketry/async-redis/pull/29. And the changes are part of async-redis since 0.5.1. We use async-redis 0.7.0 currently, however, we don't call that SentinelsClient class from anywhere in Apisonator.
• When initializing the async client with any of the sentinels configurations accepted in the StorageSync tests (storage_sync_tests.rb), the startup fails with an error caused by an unsupported URL. Which looks like nobody ever tried to support sentinels group names in URLs. The error looks like this:

undefined method `captures' for nil:NilClass (NoMethodError)

After the changes in https://github.com/3scale/apisonator/pull/350 . The same problem is manifestes as this error:

SocketError: getaddrinfo: Name or service not known

• This commit https://github.com/3scale/apisonator/commit/c37c7b3699df1a24c16e6747fd77d756651896b1 which is part of this PR https://github.com/3scale/apisonator/pull/96 explicitly says in the description that sentinels aren't supported by StorageAsync.

Notes for QE:

• The PR includes some instructions on how to test this: https://github.com/3scale/apisonator/pull/362
• Also check this notes: https://gist.github.com/mayorova/e76f78d85626b08b3476bfc5285733ac