[jdk11u-dev] RFR: 8273894: ConcurrentModificationException raised every time ReferralsCache drops referral

[Aleksey Shipilev]

Clean backport to fix the ReferralsCache bug.

Additional testing:
 - [x] Linux x86_64 fastdebug `jdk_security`

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Commit messages:
 - Backport 2166ed136917bb68f8155a25e4f4a6c5c7115566

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/455/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=455&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273894
  Stats: 5 lines in 1 file changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/455.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/455/head:pull/455

PR: https://git.openjdk.java.net/jdk11u-dev/pull/455