RFR: 8296454: System.console() shouldn't return null in jshell [v2]

[Jan Lahoda]

> Implements `System.console()` for JShell, based on JDK-8295803. It is only supported for (supported) remote agents.
> 
> When a snippet calls a Console method in the remote agent, a request to perform the task is sent to the main process, performed, and the outcomes are sent back to the remote agent, and returned from the Console method.
> 
> Please also review the CSR for the newly added API:
> https://bugs.openjdk.org/browse/JDK-8299680

Jan Lahoda 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 12 additional commits since the last revision:

 - Merge remote-tracking branch 'origin/JDK-8296454b' into JDK-8296454b
 - Merge branch 'master' into JDK-8296454b
 - Speeding up writes to the Console's Writer by not waiting for the response.
 - Cleanup.
 - Cleanup.
 - Fixing test.
 - Adding support for Charset.
 - Test when there's no JShellConsole.
 - Buffering output.
 - Improving synchronizations and fixing sending of many requests from the agent into the main process.
 - ... and 2 more: https://git.openjdk.org/jdk/compare/b71e3113...d58e53f1

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11695/files
  - new: https://git.openjdk.org/jdk/pull/11695/files/5cc0831f..d58e53f1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11695&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11695&range=00-01

  Stats: 328327 lines in 4885 files changed: 161065 ins; 120883 del; 46379 mod
  Patch: https://git.openjdk.org/jdk/pull/11695.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11695/head:pull/11695

PR: https://git.openjdk.org/jdk/pull/11695