add node discovery and node auto remove

[anderswei]

fixed several problems:

• if there is a new cassandra node joined, marina should be able to automatically establish connection pools to that node.
• if there is a cassandra node removed from cassandra cluster, marina should be able to automatically remove the connection pool to that node.
• if a node is down, marina should not route any requests to that node. requires PR https://github.com/lpgauth/shackle/pull/99