RFR: 8300268 : ServerImpl allows too many idle connections when using sun.net.httpserver.maxIdleConnections [v4]

[Darragh Clarke]

> Currently there is a race condition that can allow for too many 'idleConnections' in `ServerImpl`
> 
> This PR adds a lock to make sure only one connection can be marked Idle at a time as well as a test that consistently failed before the change but which now passes.

Darragh Clarke has updated the pull request incrementally with two additional commits since the last revision:

 - removed unused variable
 - changed test to use URIBuilder, fixed print, changed serverimpl to use sync instead of lock

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12413/files
  - new: https://git.openjdk.org/jdk/pull/12413/files/2eae924b..2bbf2876

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12413&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12413&range=02-03

  Stats: 17 lines in 2 files changed: 6 ins; 7 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/12413.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12413/head:pull/12413

PR: https://git.openjdk.org/jdk/pull/12413