RFR: 8305906: HttpClient may use incorrect key when finding pooled HTTP/2 connection for IPv6 address

[Jaikiran Pai]

Can I please get a review of this change which proposes to fix the issue reported in https://bugs.openjdk.org/browse/JDK-8305906?

As noted in that issue, when IPv6 hosts are involved, the HttpClient on certain occasions can end up caching the connection with a key which doesn't match with the key which is then used in a subsequent request against the same target host. 

The commit in this PR now wraps the IPv6 address in a square bracket consistently so that the correct key is used both during storing the connection in the pool and when looking up.

A new jtreg test has been added which reproduces this issue without the fix and verifies the fix.

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Commit messages:
 - 8305906: HttpClient may use incorrect key when finding pooled HTTP/2 connection for IPv6 address

Changes: https://git.openjdk.org/jdk/pull/13456/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13456&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305906
  Stats: 192 lines in 3 files changed: 187 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13456.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13456/head:pull/13456

PR: https://git.openjdk.org/jdk/pull/13456