Query Priority implemented.
    * Change in DriverSelector's api to facilitate explain on all drivers is called beforehand.
    * Change in QueryContext,PreparedQueryContext and addition of DriverSelectorQueryContext, AbstractQueryContext and ExplainQueryContext in order with the last point
    * Priority Decider API and implementation for Hive Driver
    * Hive Driver modifications to use Priority Decider API. Only called in async execute and surrounded with a try block.
    * Accompanying test cases