Server selector improvement

[dgolitsyn]

This PR is intended to slightly improve ServerSelector class:

1. Get rid of TreeMap creation on each call to pick() method and store servers in Int2ObjectRBTreeMap instead. Pick() is called for every segment for each query on broker side, so a lot of temp collections are created
2. Use Int2ObjectRBTreeMap instead of TreeMap and change TierSelectorStrategy interface to accept Int2ObjectRBTreeMap instead of TreeMap
3. Pick method in TierSelectorStrategy does not modify input map anymore
4. Extend ServerSelectStrategy and TierSelectorStrategy interfaces with method for multiple instance selection and use it in ServerSelector to simplify logic and keep all selection details in strategy implementation

Initial PR: metamx#58