RFR: 8330215: Trim working set for OldObjectSamples

[Markus Grönlund]

Greetings,

During the investigation around a potential memory leak, initially thought to be related to JFR and OldObjectSamples, but eventually reduced to https://bugs.openjdk.org/browse/JDK-8303767, a few minor improvements were made.

There is no need to save class unload typesets if there are no new unresolved entries for the epoch in the priority queue. 

Testing: jdk_jfr, stress testing

Thanks
Markus

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Commit messages:
 - 8330215

Changes: https://git.openjdk.org/jdk/pull/18776/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18776&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330215
  Stats: 66 lines in 4 files changed: 51 ins; 2 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/18776.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18776/head:pull/18776

PR: https://git.openjdk.org/jdk/pull/18776