Implement safepoint-aware executor service for a more fine-grained job execution control

[hubertp]

A proposition stemming from a PR review.
There we noticed that using existing java.util.concurrent.Future has its limitations when trying to perform soft and hard (interrupt) cancellations.

Having a safepoint-aware executor service (and custom Futures for jobs) could potentially simplify the logic of 2-step cancellations.

[JaroslavTulach]

• all guest code executions are handled by ThreadManager.submit
• it returns CompletableFuture
• according to its cancel method javadoc cancelling equals to completing exceptionally
• right now it doesn't stop the execution, but
• the javadoc says "Dependent CompletableFutures that have not already completed will also complete exceptionally"
• as such one could add additional exceptionallyCompose and also submit the safepoint interrupt as ThreadManager.interruptThreads() does
• update: related note at Execute yjs from insight script #11965 (comment)