Output max CPU utilization on summarize-benchmark script JSON output format

[aemous]

Description of changes:

• Add max cpu and standard deviation of max cpu to the JSON output format. This is already included in the table output format.
• Fix a rare bug in the benchmark script where the process is non-zombie during the if-check on line 72, but becomes a zombie in the try-block in lines 79-81.

Description of tests:

• Ran the benchmark and summarize scripts and confirmed the max CPU metric (and standard deviation) are now output in JSON output mode.
• After many executions of the benchmark script, I no longer observe the rare bug of an unhandled ZombieProcess exception thrown in the try block.