When `BodyHandler` isn't the first handler, request may get stuck (not processed, client times out) about vertx-web HOT 4 CLOSED

My repro is actually a bit broken - something about threads and contexts. In my actual workflow I'm using just Vert.x async processing where it repros quite well, but the minimal test case looks to be too minimal right now.

I'll fix it up in a bit, but the problem is definitely there.

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The repro is now properly showing the issue - it is running the same code twice: when setting the delay period to 0, the test passes, when setting the delay period to 10ms if gets stuck and times out.

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If you can't define the BodyHandler first, you must pause the request like this:

    router.post().handler(ctx -> {
      ctx.request().pause();
      System.out.println(Instant.now() + " Delaying before body processing");

Otherwise, all the content will be discarded before the BodyHandler gets invoked.

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@tsegismont - how about just handling the invalid situation with an error, instead of leaving the request unprocessed and unprocessable? Something like PR #2439 ?

Ideally there'll be a richer API, but if that cannot be the case - can we at least get an error?

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