Prototype: run long-spanning tasks cancellably on background in the (Java) LSP server.

[lahodaj]

@jtulach reported the code completion inside VS Code is sometimes slow for him. I was thinking of that, and experimenting, and I think at least part of the problem is that tasks that should be background tasks are not properly cancellable in the VS Code (Java) server.

I was doing experiments on:
https://github.com/openjdk/jdk/blob/master/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Attr.java

and it seemed there were three particular pain points:

• the documentSymbol, which I attempted to tackle in Fixing LspElementUtils with anonymous classes extending an enclosing class, and speeding up the StructureElement computation. #7707
• diagnostics (especially hints) computation, which can take a fairly long time to compute
• semantic coloring computation, which can also take some time to compute

Neither of these can be cancelled when the request to compute code completion. This draft is attempting to improve that, and run diagnostics and semantic coloring in a cancellable way.

It only properly cancels for Java sources at this time, for other sources runs only the CompletableFuture cancel will work. Some work on Schedulers will be needed to fully enable this for non-Java sources.

This PR is primarily for experimentation/validation that it helps, but feedback is welcome nonetheless.

I also have a separate/independent commit that adds additional logging for code completion, namely:

• logs time the CC took,
• and if it takes more 1 second, it will print a thread dump, and start self sampler.

The self sampler ran out of heap for me when running without #7707, so I would suggest to either use this additional logging with LspElementUtils fixed, or disable the self sampler part.

The additional logging commit is:
lahodaj@e3d5b2f