RFR: 8280055: JFR: Improve ObjectContext implementation

[Erik Gahlin]

Hi,

Could I have review of an enhancement that improves the ObjectContext class. For details, se bug.

I also turned the UnsignedValue class into a record class.

Testing: jdk/jdk/jfr

Thanks
Erik

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

Commit messages:
 - Update comment and copyright
 - Initial

Changes: https://git.openjdk.java.net/jdk/pull/7102/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7102&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280055
  Stats: 43 lines in 2 files changed: 18 ins; 11 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7102.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7102/head:pull/7102

PR: https://git.openjdk.java.net/jdk/pull/7102