RFR: 8287318: ConcurrentModificationException in sun.net.httpserver.ServerImpl$Dispatcher

[Jaikiran Pai]

Can I please get a review of this change which addresses the issue noted in https://bugs.openjdk.java.net/browse/JDK-8287318?

The `ServerImpl` has a `Dispatcher` thread which uses a `Selector` to select keys of interested. The selected keys is then iterated over and each key is removed through the iterator. This is fine as long as the selector isn't then used to invoke select operation(s) while the iterator is still in use. Doing so leads to the underlying Set being potentially modified with updates to the selected keys. As a result any subsequent use of the iterator will lead to `ConcurrentModificationException` as seen in the linked issue.

The commit here fixes that by creating a copy of the selected keys and iterating over it so that any subsequent select operation on the selector won't have impact on the Set that is being iterated upon. 

No new tests have been added given the intermittent nature of this issue. Existing tier1, tier2 and tier3 tests passed without any related failures, after this change.

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Commit messages:
 - 8287318: ConcurrentModificationException in sun.net.httpserver.ServerImpl

Changes: https://git.openjdk.java.net/jdk/pull/8898/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8898&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287318
  Stats: 10 lines in 1 file changed: 6 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8898.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8898/head:pull/8898

PR: https://git.openjdk.java.net/jdk/pull/8898