Summary: For compactions benchmarks (both level and universal) we'll use `--max_write_buffer_number=4`. For all the other benchmarks which don't customize the value of `--max_background_flushes` we'll continue using `--max_write_buffer_number=8`.
Test Plan:
To validate basic correctness and command-line options:
```
cd ~/rocksdb
NKEYS=10000000 ./tools/run_flash_bench.sh
```
Reviewers: MarkCallaghan
Reviewed By: MarkCallaghan
Subscribers: andrewkr, dhruba
Differential Revision: https://reviews.facebook.net/D55497
90aff0c444