RFR: 8260267: ZGC: Reduce mark stack usage

[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.

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Commit messages:
 - ZGC: Mark before push if mark stack usage is high
 - ZGC: Reduce mark stack usage

Changes: https://git.openjdk.java.net/jdk/pull/3460/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3460&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260267
  Stats: 285 lines in 19 files changed: 192 ins; 41 del; 52 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