Refactor class CheckIntegrity(Thread) to use local-space variables

[Zerpet]

We thought that class CheckIntegrity was using local variables since we created references in the __init__() constructor to the global variables. However, those are just references to the global object, not an actual private copy.

At the moment there is no risk or race-conditions because we don't modify the object with the thread information after initializing and starting the threads. However, we should do this thread-safe and implement thread-local variables.