Alias _client to client in 3.3.x for smoother transition to 4.0

[djanowski]

sidekiq/sidekiq#3256 (comment)

[mperham]

FYI, I use the client to get access to id attribute, so I can show the URL of the server:

    def redis_connection
      Sidekiq.redis { |conn| conn.client.id }
    end

If there's a better way to get access to this in 3.3.3+, I'm listening.

[djanowski]

3.3.4 and 4.0.1 include Redis#connection.