Handle the noop infinite req/s case in reports.
Handle the noop infinite req/s case in reports.

When generating reports for a --noop run of ssbench, each request
takes 0 time (start and stop times are the same). When a worker has
only 1 request, this results in a ZeroDivisionError (1 request/0
seconds). The patch changes the behavior to print "inf" in such cases.

Fixes #96

swiftstack/ssbench,swiftstack/ssbench