Remove unnecessary thread pool recreation in DruidCoordinator

[ArvinZheng]

This thread pool is recreated every time when DutiesRunnable runs which is a waste of resources and also I don't think people will change this config item balancerComputeThreads frequently, we could cache current value and recreate the thread pool only when it's necessary (config value change)

druid/server/src/main/java/org/apache/druid/server/coordinator/DruidCoordinator.java

Line 665 in d36a0f6

ListeningExecutorService balancerExec = null;