RFR: 8300268 : ServerImpl allows too many idle connections when using sun.net.httpserver.maxIdleConnections [v3]

[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 one additional commit since the last revision:

  addressed comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12413/files
  - new: https://git.openjdk.org/jdk/pull/12413/files/c9807613..2eae924b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12413&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12413&range=01-02

  Stats: 4 lines in 2 files changed: 2 ins; 1 del; 1 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