Add more command-line options to swift-bench.
Add more command-line options to swift-bench.

These settings:
  1. are already exposed via config file
  2. were not exposed as command-line options, and
  3. are things I wanted on the command line while benchmarking recently.

I didn't include short options for --(get|put|delete)-concurrency
since I couldn't think of good single-letter ones, and optparse won't
take "-gc", "-pc", or "-dc".

Change-Id: I0bd7c7ae2892244eed37b8c10c9357dc7e1190d3

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