Redis: Adds support for checking length of more types

I saw the list length checking feature (#945) was added in the latest version of the check, and thought that I could add some of the code I have from a check I've made to do more or less the same. This adds support for checking other list-like types, namely sets, sorted sets and hashes.

Also, since a key disappears in Redis whenever a list-like item is emptied, it should send a value of 0 to Datadog instead of complaining of the key being missing.