Add an extension of LBHttpSolrClient for the better load-balancing.

We propose a new type of LBHttpSolrClient which uses UCB algorithm to select a next candidate from replicas.
This new client uses past responses (currently, only QTime) to calculate the priority of each replica, and selects the best one for keeping the stability of latencies.

Our experiment shows that the new client performed better than LBHttpSolrClient with failure cases such as the performance of one node goes down by some reasons.
Please see attached PDF for more details.