RFR: 8282131: java.time.ZoneId should be a sealed abstract class

[Naoto Sato]

Refactoring `java.time.ZoneId` class to be a sealed class. A CSR has also been drafted.

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Commit messages:
 - 8282131: java.time.ZoneId should be a sealed abstract class

Changes: https://git.openjdk.java.net/jdk/pull/7625/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7625&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282131
  Stats: 10 lines in 1 file changed: 0 ins; 4 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7625.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7625/head:pull/7625

PR: https://git.openjdk.java.net/jdk/pull/7625