Warmers: Have an explicit warmer thread pool
Have an explicit threadpool warmer that is dedicated to execute warmers. Currently, it uses the search threadpool, which does not work well since the number of concurrent searches should be separate from the number of concurrent warmers allows, also the characteristics of the search pool (for example, bounded queue_size) might not fit well with how warmers should be executed (they should not be "rejected").

closes #2815