RFR: 8304818: Prune HttpURLConnection cache when corresponding Authenticator is garbage collected [v5]

[Michael McMahon]

> Hi,
> 
> Can I get a review for this please? This change uses a Cleaner to check when java.net.Authenticator instances become unreachable, and then prunes the authentication cache for entries that correspond to that Authenticator and therefore cannot be used again. This stops the authentication cache from growing in size unbounded. Note, this only happens in the probably unusual case where more than one Authenticator object is used. Normally there is no reason to use more than one instance.
> 
> Thanks,
> Michael.

Michael McMahon has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:

 - update
 - Merge branch 'master' into auth
 - updated implementation and reimpl of AuthenticatoKeys
 - review updates
 - test update
 - revert removed methods (used in tests)
 - initial impl

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13159/files
  - new: https://git.openjdk.org/jdk/pull/13159/files/610ef0bf..db24a545

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13159&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13159&range=03-04

  Stats: 24931 lines in 678 files changed: 11918 ins; 8816 del; 4197 mod
  Patch: https://git.openjdk.org/jdk/pull/13159.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13159/head:pull/13159

PR: https://git.openjdk.org/jdk/pull/13159