RFR: 8255235: ZGC: Allocate and initialize forwarding data structures in parallel [v3]

[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 incrementally with three additional commits since the last revision:

 - Add assert
 - Merge
 - Assert that all allocated memory is used

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/805/files
  - new: https://git.openjdk.java.net/jdk/pull/805/files/6f340aa1..e5067758

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=805&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=805&range=01-02

  Stats: 12 lines in 3 files changed: 12 ins; 0 del; 0 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