RFR: 8255235: ZGC: Allocate and initialize forwarding data structures in parallel [v4]

[Per Liden]

> As a continuation of JDK-8255234, we can speed things up even more by allocating/initializing forwarding data structures in parallel. For example, testing using BigRamTester shows the time for "Concurrent Select Relocation Set" phase drop by ~50% when using 4 threads.
> 
> This PR depends on "8255234: ZGC: Bulk allocate forwarding data structures", which is a separate PR (https://github.com/openjdk/jdk/pull/804).

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

 - Merge master
 - Add assert
 - Merge
 - Assert that all allocated memory is used
 - Merge
 - Assert that allocations never fail
 - 8255235: ZGC: Allocate and initialize forwarding data structures in parallel
 - 8255234: ZGC: Bulk allocate forwarding data structures

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Changes: https://git.openjdk.java.net/jdk/pull/805/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=805&range=03
  Stats: 261 lines in 11 files changed: 112 ins; 93 del; 56 mod
  Patch: https://git.openjdk.java.net/jdk/pull/805.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/805/head:pull/805

PR: https://git.openjdk.java.net/jdk/pull/805