SI-7438 Avoid a performance bug in our JSR166 redistribution

Use `execute` unconditionally, rather than `fork`, which in some
fairly easy to provoke circumstances fails to utilize all available
cores.

The longer term fix is the update to the latest version of the JSR133
code, and then the restore the use of `fork`.

Benchmark and results:

  https://gist.github.com/retronym/5495585

Review by @phaller.

Even if we do decide to update FJ in 2.10.2, I think that we should start with this commit. This gives us better "rollback-ability" of the FJ update in case of some unexpected problem.