Make sure the the verifiers cleanup any child process when stopped

[dinoboff]

Under heavy load, verifier container may fail to be removed. It might be because they create orphan process when getting removed.

• [x] assert the verifiers stop their child process when getting stop.
• [x] assert the verifier daemon stop a verifier before removing it.

[dinoboff]

The Java verifier should be fine; it runs the verifier code in a thread and doesn't spawn any child process.

[dinoboff]

@SingaporeClouds It fixes the issue with the server freezing under heavy load. I can run benchmark without the generating zombie containers.