RFR: 8260267: ZGC: Reduce mark stack usage [v6]

[Per Liden]

> Under some conditions, striped marking can cause excessive mark stack memory usage. For example, when the object graph has one or more N:1 relations ships, where N is a large number.
> 
> To avoid pushing mark stack entries to already marked objects, and thus reduce mark stack memory usage, this patch adds does two things:
>  * GC threads will under some conditions mark the object before pushing, else (if the condition to mark failed)
>  * GC threads will check if the object is already marked before pushing.
> 
> The conditions for marking before pushing is:
>  * We are using a single stripe, in which cases striped marking is essentially disabled anyway.
>  * We see excessive mark stack usage. In other words, the mark stack allocator indicates "high usage".
> 
> To make this easier to review, this PR contains two commits:
>  * `ZGC: Reduce mark stack usage` - This patch adds the infrastructure needed to mark before push, but only does so if we are using a single stripe.
>  * `ZGC: Mark before push if mark stack usage is high` - This patch adds the condition to mark before push if the mark stack allocator indicates "high usage". As a bonus, this patch will also uncommit memory used for mark stacks after the marking has completed.
> 
> Testing:
>  * Tier1-7
>  * SPECjbb2015, score and marking times unaffected
>  * casparcwang at tencent.com reports that this patch, in combination with PR #3455, solves the problem for Apache Zookeeper.

Per Liden has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains ten commits:

 - Merge master
 - Print mark stack memory usage
 - Shrink to currently used
 - Always mark before push
 - Review comments from StefanJ
 - Review comments from StefanJ and Albert
 - Merge master
 - ZGC: Mark before push if mark stack usage is high
 - ZGC: Reduce mark stack usage

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Changes: https://git.openjdk.java.net/jdk/pull/3460/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3460&range=05
  Stats: 308 lines in 20 files changed: 204 ins; 53 del; 51 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3460.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3460/head:pull/3460

PR: https://git.openjdk.java.net/jdk/pull/3460