RFR: JDK-8283415: Update java.lang.ref to use sealed classes [v2]

[Joe Darcy]

> Another refactoring to use sealed classes where possible, this time in the java.lang.ref package.
> 
> Copyright dates will be updated, if needed, before a push.

Joe Darcy has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Update copyrights.
 - Merge branch 'master' into JDK-8283415
 - JDK-8283415: Update java.lang.ref to use sealed classes

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7874/files
  - new: https://git.openjdk.java.net/jdk/pull/7874/files/8dc83e5f..6382c607

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7874&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7874&range=00-01

  Stats: 1302 lines in 71 files changed: 785 ins; 314 del; 203 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7874.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7874/head:pull/7874

PR: https://git.openjdk.java.net/jdk/pull/7874