Issue #200: cassandraTable optimisations

Replace stmt.execute to asyncExecute for performance
If table fit in one spark partition, skip all unneccessary ring locality optimisations
Fixes #200 
Take a look at the code and docs
http://www.datastax.com/documentation/developer/java-driver/1.0/common/drivers/reference/connectionsOptions_c.html
They do control concurrency. The only possible problem I see here is:
Before the fix connection timeout where applied to each individual request. Now It is applied to all concurrent requests. Thus the fail chance should increased