[funfuzz] Rewrite fork_join using concurrent.futures

[nth10sd]

After #135 landed, Windows is unable to spawn multiple processes when fuzzing. When I manually bypass the call to fork_join, fuzzing works as expected with a single thread.

We can either:

• Fix the current fork_join code
• Rewrite using concurrent.futures, and we can import the futures package to make it work on Python 2.7.

I'm leaning towards the second option, after getting #138 in place. Edit: it may be more important to unbreak this first.

[nth10sd]

The output with stack (when fixated to one process) is:

=== Waiting for child #0 (8448) to finish... ===
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "c:\mozilla-build\python\lib\multiprocessing\forking.py", line 380, in main
    prepare(preparation_data)
  File "c:\mozilla-build\python\lib\multiprocessing\forking.py", line 509, in prepare
    '__parents_main__', file, path_name, etc
  File "c:\mozilla-build\python\lib\site-packages\funfuzz\bot.py", line 23, in <module>
    from .js import build_options
ValueError: Attempted relative import in non-package
=== Child process #0 exited with code 1 ===

Apparently this is because multiprocessing in Python 2 does not play nice with explicit relative imports (whereas Python 3.4+ seems fine). It had been added in Python 2.6 which is now end-of-life.

In conclusion, our custom multiple process fuzzing implementation from Sep 2012 (fork_join) does not work on Windows with the new module packaging system.

[nth10sd]

Another way is to wait this out till our Python 3 migration is complete, where the issue here will be fixed by itself by only using Python 3 on Windows.