Benchmarks: refactored JSON logging format

Instead of writing the whole log as a valid JSON object, it is much
easier to write one JSON object per line. This can be processed in
Python with the Pandas package using the following code:

df = pandas.read_json(open("foo.log", "r"), orient="records", lines=True)

Also removed the outputMode parameter from the Benchmark and Logging
classes, since it was pretty useless.