Functions
    Methods to be implemented
    setParameters(Collection)
    
        
    execute(prevResult,currentSampler)
    Note that either or both of the parameters may be null.
    
    Calling sequence
    When the test plan is prepared for running, one instance of the class is created for each occurrence
    of a function call. The setParameters() method is then called on each instance.
    Once the test is running, the execute method can be called by any thread, and is
    therefore synchronized.
    
    This is unlike most of (all?) the JMeter test elements, which are created for each thread.
    
    Any context that needs to be maintained for a thread must be done using ThreadLocal or similar.