Optimize loop in range client program

[zobront]

Currently, the main loop of the range client program calls driver.produce_payloads(), which calls step() on the pipeline, and then loops over all available attrs in the pipeline and returns a vector of all of them.

However, after calling step() once only one attr at a time is made accessible. So the vector will have a length of at most one.

This allows us to slightly simplify the logic, by just returning an Option for a single payload attribute, and then executing that in the main loop without having to iterate over the vector.

[ratankaliani]

Added in #68