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

[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.

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Commit messages:
 - 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
 - basic functional test
 - change referent access
 - expand test
 - ... and 2 more: https://git.openjdk.java.net/jdk/compare/f0b11940...b1e645b1

Changes: https://git.openjdk.java.net/jdk/pull/1609/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1609&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256167
  Stats: 52 lines in 12 files changed: 7 ins; 10 del; 35 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