Add cpu usage to regression benchmarks (3rd attempt)
Summary:
Tested by running rocks/tools/debug_regression_test.sh and verifying the local output:
```
cat local/rocks_regression_tests/OPTIONS-viewstate-66-1262-5000/2017-04-13-11-34-51/SUMMARY.csv
commit id, benchmark, user@host,num-dbs,key-range,key-size,value-size,compress-rate,ops-per-thread,num-threads, cache-size,flushes,compactions,ops-per-s, p50, p75, p99, p99.9, p99.99,debug,real-sec,user-sec,sys-sec
d2dce5611a
, readrandom, root@localhost, 12, 5000, 66, 1262, 50, 312, 16, 1073741824, 4, 16, 138458, 9380, 11530, 55200, 16803200, 32504000, 1, 0, 0, 0
d2dce5611a
, readwhilewriting, root@localhost, 12, 5000, 66, 1262, 50, 312, 16, 1073741824, 4, 16, 104511,
Closes https://github.com/facebook/rocksdb/pull/2157
Differential Revision: D4909238
Pulled By: maysamyabandeh
fbshipit-source-id: dc7bb8569c3c33b9f7c4ba47a757b24d27bb3b31