Cassandra: allow HostSelectionPolicy to be set to TokenAwarePolicy

[tomwilkie]

We should allow the HostSelectionPolicy to be configured. https://godoc.org/github.com/gocql/gocql#PoolConfig

The TokenAwarePolicy will yield the best performance.

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[pracucci]

Still valid

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[bboreham]

Help wanted: need someone with knowledge of Cassandra APIs