RFR: 8256167: Convert JDK use of `Reference::get` to `Reference::refersTo` [v2]

[Mandy Chung]

> This patch replaces some uses of `Reference::get` to `Reference::refersTo` to avoid keeping a referent strongly reachable that could cause unnecessary delay in collecting such object.   I only made change in some but not all classes in core libraries when working with Kim on `Reference::refersTo`.    The remaining uses are left for the component owners to convert at appropriate time.

Mandy Chung has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 15 commits:

 - revise matchsKey to check equality if the reference is not cleared
 - fix typo
 - update copyright end year and fixup grammatical nit
 - Merge branch 'master' of https://github.com/openjdk/jdk into refersTo
 - 8256167: Convert JDK use of  to
 - Merge branch 'master' of https://github.com/openjdk/jdk into refersTo
 - Merge branch 'master' of https://github.com/openjdk/jdk into refersTo
 - minor cleanup
 - Merge branch 'refersto' of https://github.com/kimbarrett/openjdk-jdk into refersTo
 - improve refersTo0 descriptions
 - ... and 5 more: https://git.openjdk.java.net/jdk/compare/d8ac76fa...72947eb3

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Changes: https://git.openjdk.java.net/jdk/pull/1609/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1609&range=01
  Stats: 60 lines in 12 files changed: 7 ins; 11 del; 42 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1609.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1609/head:pull/1609

PR: https://git.openjdk.java.net/jdk/pull/1609