RFR: 8305906: HttpClient may use incorrect key when finding pooled HTTP/2 connection for IPv6 address [v5]

[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.

Jaikiran Pai has updated the pull request incrementally with one additional commit since the last revision:

  simplify the keyFor method to accept the entire HttpRequestImpl instance

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/13456/files
  - new: https://git.openjdk.org/jdk/pull/13456/files/6db0ea8c..13821d88

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13456&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13456&range=03-04

  Stats: 8 lines in 2 files changed: 2 ins; 1 del; 5 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