Angora termination

[GrosQuildu]

If we uncomment code in do_restart, then fuzzer processes will be killed and started again (so resumed with the same output directory) after 20 seconds of fuzzing. This should work smoothly, as all fuzzers implement resuming.

But according to the tests/test_fuzzers_sync.py, doing so for Angora fails. Probably because angora spawns child processes and we don't correctly kill them. Then both child processes and new Angora process try to access output directory and that kills the new process (seems like angora has some file locking implemented).

We implement process termination in cleanup method. It must have some bug because it doesn't correctly kill Angora. Debugging needed.