RFR: 8257073: ZGC: Try forward object before retaining page

[Per Liden]

This is a follow up to @albertnetymk's comment in the review of #1228, where he suggested that we could try to forward the object before we go ahead and retain the page. This could be an optimization in two cases:
1) A mutator would avoid a relocation stall in case the page is claimed, but the object the mutator is interested in has already been relocated.
2) The worker thread detaching the page would not have to wait for a mutator that is just forwarding an object.

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Commit messages:
 - 8257073: ZGC: Try forward object before retaining page

Changes: https://git.openjdk.java.net/jdk/pull/1431/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1431&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257073
  Stats: 26 lines in 3 files changed: 11 ins; 10 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1431.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1431/head:pull/1431

PR: https://git.openjdk.java.net/jdk/pull/1431