JMeter tests cannot started

[gnomeby]

After some interface changes in TimeChopper the JMeter tests starts failing.

<class 'TypeError'>: object of type 'generator' has no len() self.recent_ts = [0] * len(self.sources)

TimeChopper(self._read_stat_queue(), 3)) TypeError: __init__() takes 2 positional arguments but 3 were given

[gnomeby]

Proposed fix #865