RFR: JDK-8310913 Move ReferencedKeyMap to jdk.internal so it may be shared [v7]

[Jim Laskey]

> java.lang.runtime.ReferencedKeyMap was introduced to provide a concurrent caching scheme for Carrier objects. The technique used is generally useful for a variety of caching schemes and is being moved to be shared in other parts of the jdk. The MethodType interning case is one example.

Jim Laskey has updated the pull request incrementally with one additional commit since the last revision:

  Update ReferencedKeyTest.java

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14684/files
  - new: https://git.openjdk.org/jdk/pull/14684/files/8eff9183..bb943b6f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14684&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14684&range=05-06

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14684.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14684/head:pull/14684

PR: https://git.openjdk.org/jdk/pull/14684