RFR: 8268372: ZGC: dynamically select the number of concurrent GC threads used

[Albert Mingkun Yang]

This patch adds support for dynamically selecting the number of GC threads to ZGC.

1. The number of GC threads (in `[1, ConcGCThreads]`) is dynamically selected for each GC cycle.
2. Increase the default value of ConcGCThreads from 12.5% to 25% of total CPUs.

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Commit messages:
 - ZGC: Dynamic number of GC threads

Changes: https://git.openjdk.java.net/jdk/pull/4410/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4410&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268372
  Stats: 786 lines in 29 files changed: 415 ins; 192 del; 179 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4410.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4410/head:pull/4410

PR: https://git.openjdk.java.net/jdk/pull/4410