RFR: 8273894: ConcurrentModificationException raised every time ReferralsCache drops referral

[Jaikiran Pai]

Can I please get a review for this change which proposes to fix the issue noted in https://bugs.openjdk.java.net/browse/JDK-8273894?

Given the nature of the code in `ReferralsCache`, I haven't been able to add a new jtreg test case to reproduce the issue and verify this fix. Just to be sure that this change indeed fixes a reproducible issue, I replicated that code in a pretty trivial Java application and then verified a similar fix.

Existing tests in `test/jdk/sun/security/krb5/auto/` continue to pass after this change.

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Commit messages:
 - 8273894: ConcurrentModificationException raised every time ReferralsCache drops referral

Changes: https://git.openjdk.java.net/jdk/pull/5632/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5632&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/jdk/pull/5632.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5632/head:pull/5632

PR: https://git.openjdk.java.net/jdk/pull/5632