Provide ExecutorService.asCoroutineDispatcher that returns a closeable dispatcher

[elizarov]

This issue in inspired by this SO question: https://stackoverflow.com/questions/49217668/shut-down-the-underlying-executor-of-executorcoroutinedispatcher

The idea is to provide ExecutorService.asCoroutineDispatcher extension in addition to Executor.asCoroutineDispatcher that would return a subclass of CoroutineDispatcher that also implements Closeable interface and delegates close to shutdown of the underlying ExecutorService.