RFR: 8255235: ZGC: Allocate and initialize forwarding data structures in parallel [v2]

[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 two additional commits since the last revision:

 - Merge
 - Assert that allocations never fail

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/805/files
  - new: https://git.openjdk.java.net/jdk/pull/805/files/b254c90d..6f340aa1

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=805&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=805&range=00-01

  Stats: 18 lines in 1 file changed: 0 ins; 15 del; 3 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