Add --writes_per_second rate limit, print p99.99 in histogram

Summary:
Adds the --writes_per_second rate limit for the readwhilewriting test.
The purpose is to optionally avoid saturating storage with writes & compaction
and test read response time when some writes are being done.

Changes the histogram code to also print the p99.99 value

Task ID: #

Blame Rev:

Test Plan:
make check, ran db_bench with it

Revert Plan:

Database Impact:

Memcache Impact:

Other Notes:

EImportant:

- begin *PUBLIC* platform impact section -
Bugzilla: #
- end platform impact -

Reviewers: haobo

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D10305

util/histogram.cc

@@ -152,8 +152,10 @@ std::string HistogramImpl::ToString() const {
            (num_ == 0.0 ? 0.0 : min_), Median(), max_);
   r.append(buf);
   snprintf(buf, sizeof(buf),
-           "Percentiles: P50: %.2f P75: %.2f P99: %.2f P99.9: %.2f\n",
-           Percentile(50), Percentile(75), Percentile(99), Percentile(99.9));
+           "Percentiles: "
+           "P50: %.2f P75: %.2f P99: %.2f P99.9: %.2f P99.99: %.2f\n",
+           Percentile(50), Percentile(75), Percentile(99), Percentile(99.9),
+           Percentile(99.99));
   r.append(buf);
   r.append("------------------------------------------------------\n");
   const double mult = 100.0 / num_;