6944584: Improvements to subprocess handling on Unix
Summary: use thread pool for reaper thread; move most I/O operations out of reaper thread
Reviewed-by: michaelm, hiroshi
^build/
^dist/
/nbproject/private/
^src/share/tools/hsdis/build/
^src/share/tools/IdealGraphVisualizer/[a-zA-Z0-9]*/build/
^src/share/tools/IdealGraphVisualizer/build/
^src/share/tools/IdealGraphVisualizer/dist/