RFR: 8300268 : ServerImpl allows too many idle connections when using sun.net.httpserver.maxIdleConnections [v2]

[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:

 - use right boolean
 - addressed comment and made test less strict

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12413/files
  - new: https://git.openjdk.org/jdk/pull/12413/files/fc9b3f42..c9807613

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12413&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12413&range=00-01

  Stats: 19 lines in 2 files changed: 4 ins; 2 del; 13 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