Support Redis cluster mode with fallback on Master failures

[aaronleesmith]

Currently, Bloom supports a single Redis host / port. When Redis is run in a clustered mode with multiple masters, Bloom is unable to consider another Redis node to talk to for continued use upon the failure of a master node.

Libraries like ioredis support this kind of functionality (https://github.com/luin/ioredis/blob/master/API.md#Cluster).