firewall: thread safe api

[jerome-jutteau]

Bricks are not threads safe meaning that a single brick should not be accessed at the same time. But we must be able to call all api functions in different contexts in a safe way (like reload two different firewall at the same time). Some test shows that is a segfault while using firewall api in different threads. We must add tests who call some firewall's api in multiple thread and track any bug.