RFR: 8254562: ZGC: Remove ZMarkRootsTask

[Stefan Karlsson]

On Mon, 12 Oct 2020 08:04:52 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> After introducing concurrent stack scanning, we don't need to mark through any roots during the mark start pause.
> Remove the code.
> Note that the old code passed in `false` to `_roots(false /* visit_jvmti_weak_export */)`. This has the effect that no
> roots are visited by the iterator: void ZRootsIterator::oops_do(ZRootsIteratorClosure* cl) {
>   ZStatTimer timer(ZSubPhasePauseRoots);
>   if (_visit_jvmti_weak_export) {
>     _jvmti_weak_export.oops_do(cl);
>   }
> }

Looking at this some more I figured that we could clean up the rest of the ZRootIterator usages. The JVMTI cleaning
isn't really a root, but rather a way for us to update the JVMTI datastructure after the objects have moved (update
their identities).

So, the last patch:
1) Removes the unnecessary spawning of parallel threads in ZRelocate::start. The JVMTI roots are only visited in a
single-threaded operation. 2) Removed/replaced ZRootsIterator. Neither ZVerify, nor ZHeapIterator, really used it,
because they implicitly sent in `false` to the constructor (see above comment). ZRelocate now performs a direct calls,
which makes it easier to understand, IMHO.

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PR: https://git.openjdk.java.net/jdk/pull/601