RFR: 8358693: [leyden] Bootstrapping circularity leads to never starting preload threads

[Aleksey Shipilev]

This relates to [JDK-8358690](https://bugs.openjdk.org/browse/JDK-8358690), but we want to make sure Leyden preload runs until we have a fuller fix.

Additional testing:
 - [x] Ad-hoc perf tests
 - [ ] Linux x86_64 server fastdebug, `runtime/cds`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/leyden/pull/75/files
  Webrev: https://webrevs.openjdk.org/?repo=leyden&pr=75&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358693
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/leyden/pull/75.diff
  Fetch: git fetch https://git.openjdk.org/leyden.git pull/75/head:pull/75

PR: https://git.openjdk.org/leyden/pull/75