CassandraConnector shares core driver Session objects. Previously it sha...

...red only Cluster objects and a separate Session was open per every Spark task. This had a significant performance overhead in terms of CPU, networking and memory use, because every Session object manages an internal pool of connections to all cluster nodes. Fixes #41 and #53.
I am wondering if we should return a cluster proxy here so that it handles new session creation... wdyt?