RFR: 8329593: Drop adjustments to target parallelism when virtual threads do I/O on files opened for buffered I/O [v2]

[Alan Bateman]

> This change drops the adjustments to the virtual thread scheduler's target parallelism when virtual threads do file operations on files that are opened for buffered I/O. These operations are usually just too short to have any benefit and may have a negative benefit when reading/writing a small number of bytes. There is no change for read/write operations on files opened for direct I/O or when writing to files that are opened with options for synchronized I/O file integrity (O_SYNC/O_DSYNC and equivalents). Sergery Kuksenko is polishing benchmarks that includes this area, this is for a future PR.
> 
> In addition, the blocker mechanism is updated to handle reentrancy (as can happen if debugging code is added to ForkJoinPool) and preemption when compensating (as can happen when substituting a heap buffer with a direct buffer in some I/O operations).  This part is a pre-requisite to the changes to better support object monitor there are more places where preemption is possible and this quickly leads to unbalanced begin/end.
> 
> The changes have been baking in loom repo for some time.

Alan Bateman 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 five additional commits since the last revision:

 - Merge
 - Sync up from loom repo, update copyright headers
 - Merge
 - Merge
 - Initial commit

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18598/files
  - new: https://git.openjdk.org/jdk/pull/18598/files/a21a8d6b..0d99fe0c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18598&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18598&range=00-01

  Stats: 35845 lines in 976 files changed: 19000 ins; 10527 del; 6318 mod
  Patch: https://git.openjdk.org/jdk/pull/18598.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18598/head:pull/18598

PR: https://git.openjdk.org/jdk/pull/18598