Add host selector strategy configuration

When we switched from round-robin to least outstanding host selector we noticed a drop in request execution time by factor: 
mean: ~ 4.
95th percentile: ~ 10
99th percentile: ~ 12 

This is with a 60 node Cassandra cluster.
~95 % read requests, ~5 % write requests

Connection pool configuration: 
ConnectionPoolType == ConnectionPoolType.TOKEN_AWARE
DiscoveryType == NodeDiscoveryType.RING_DESCRIBE
MaxConnsPerHost == InitConnsPerHost = 40
MaxTimeoutWhenExhausted = 0 (fast fail)