Make client-side Token Aware Balancing possible with Global Indexes

This is a follow up ticket for CASSANDRA-6477, to consider enabling client-side Token Awareness Balancing with Global Indexes, a it would be a pity to loose this efficient optimization when using these new indexes. My original comment was:

Something that hasn't been mentioned so far in this thread is client side's Token Aware Balancing. So far when a query relies on a Secondary Index, it doesn't have any partition key specified in its where clause which means that the driver will fall back on the underlying balancing policy, which is DC aware round robin by default in the DS Java Driver. That's the appropriate behavior as there's no node that can be better than another as a coordinator in this situation.
With Global Indexes, in order for the Driver to still be able to perform Token Aware Balancing, it'll need to be able to figure out which index will be used, which doesn't seems to be always trivial in the above examples, especially if several columns and/or indexes are involved in the WHERE clause. So here we might need to include an extra information about it in the Prepare response message of the Native Protocol, if the indexes to be involved can be figured out at Prepare-time.