Allow to dynamically update thread pool settings. The settings can be updated using Cluster Update Settings API. Both pool type and pool parameters can be changed dynamically. Minor changes, such as number of threads or queue size, are made to the existing thread pool executor. To apply major changes such as thread pool type or queue type changes, Elasticsearch replaces the old thread executor with a new executor. When this happens Elasticsearch creates a new thread pool first and starts executing all new tasks using the new pool, meanwhile all tasks that are currently executed in the old pool are allowed to finish before the old thread pool is stopped.