this PR adds a JavaScriptWorkerSelectStrategy which allows defining
arbitrary logic for selecting workers to run task using a JavaScript
function.
This gives users full control to implement complex worker selection
strategies based on task attributes.
more tests and a complex javascript config
fix for java8 modify for nashorn compatibility
which could cause Guice duplicate binding errors.
Add unit test to replicate duplicate extension issue. Update documents. Add an package accessible getter for loadersMap for testing only.
Notice that extensions explicitly specified in druid.extensions.coordinates have a higher priority than ones included in the classpath.
Extension modules that don't have a canonical class name will be ignored.
