RFR: 8369656 : Calling CompletableFuture.join() could execute task in common pool

[Viktor Klang]

Addresses the regression regarding pool assistance and includes a regression test for `CompletableFuture::get()`, `CompletableFuture::get(timeout, unit)`, and `CompletableFuture::join()`

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

Commit messages:
 - Reinstating individual imports
 - JDK-8369656 - ensure that caller-assistance only occurs if the pool is shared between caller and callee

Changes: https://git.openjdk.org/jdk/pull/27800/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27800&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369656
  Stats: 47 lines in 2 files changed: 43 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/27800.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27800/head:pull/27800

PR: https://git.openjdk.org/jdk/pull/27800