Max throughput performance test

A test to determine the max sustainable throughput for an instance of the driver under varying configuration.

Allow to specify a min, max, step, and waiting period.
n = min
while max throughput is unknown:
send n requests per second for the duration of the waiting period
if no failures or timeouts are seen:
n = n + step
else:
n = n - step
break

See PYTHON-1206

This should give us our max sustainable throughput. This should be incorporated into the performance harness run on fallout.