Support multiple workers

[GiedriusS]

Because of https://github.com/ncabatoff/process-exporter/blob/master/collector/process_collector.go#L206 it means that if multiple requests come in at the same time, the latency is additive e.g. the 2nd request waits until the 1st one completes. A parameter needs to be added to spawn multiple workers && the Tracker struct must avoid sharing state.