Add the support of using Redis as an alternative storage

[jihuayu]

In this PR I add the support of using Redis as an alternative storage.

In electLoop function I don't used Redis subscribe command, because the expiration of the key is not real-time. I have used get command every 2s to check the key status. Others is same as Zookeeper storage impl.

In addition, I have made adjustments to the configuration structure to make it clearer. And, I have written compatibility code for the old version of the configuration structure. I think we can keep this compatibility code until the 1.0 release.