RFR: 8330814: Cleanups for KeepAliveCache tests [v3]

[Christoph Langer]

> While working in that area I found some potential for cleanup of a few tests.
> 
> Most notably:
> 
> B5045306.java:
> - does not need to run in othervm
> - the executor service that it uses should be shut down eventually to free resources
> 
> B8291637.java:
> - use just one instead of two test VM invocations
> KeepAliveTimerThread.java:
> call to grp.destroy() at the end is pointless (API is void & deprecated for removal)
> 
> Generally:
> The deprecated URL constructor is used. It can be switched to the handy URIBuilder
> Some more try with resources here and there

Christoph Langer 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 five additional commits since the last revision:

 - Merge branch 'master' into keepalivetests
 - Jaikiran's comments
 - Update test/jdk/sun/net/www/http/KeepAliveCache/B5045306.java
   
   Co-authored-by: Andrey Turbanov <turbanoff at gmail.com>
 - Copyright year
 - JDK-8330814

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18884/files
  - new: https://git.openjdk.org/jdk/pull/18884/files/92bddff8..a89416f0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18884&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18884&range=01-02

  Stats: 9885 lines in 316 files changed: 6019 ins; 2755 del; 1111 mod
  Patch: https://git.openjdk.org/jdk/pull/18884.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18884/head:pull/18884

PR: https://git.openjdk.org/jdk/pull/18884