Add maximum worker control

[ppfeister]

Closes #2059

Argument --workers is added to Sherlock, default 20, allowing the user to specify the maximum number of workers. If the target list is shorter than the maximum, it will not create more workers than targets. No upper limit applied to argument, but the value should be type safe.

Validated with values both higher and lower than the default.

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On most capable machines, the bottleneck is the response time of individual targets rather than process time. This can't help very much with response time.

It does allow you to better control worker count in very-low-power situations or in those that have consistently delayed results (to avoid stacked delays). Users using --tor would likely see the most benefit, since those loading times can stack.

Discretionary feature PR