Lazily initialize redis clients, reduce side effects of `Queue` instantiation

Created by: marshall007

Fixes #526 (closed)

This PR ensures that the client and eclient redis connections are not instantiated until they are absolutely needed. This in turn reduces the number of bound event listeners by default.

Additionally, I have added a skipVersionCheck option which, when true, avoids creating a redis connection and sanity checking the MINIMUM_REDIS_VERSION in the constructor.