Use the elapsed time (instead of the per-thread time) to compute ops/sec.

Summary:
Task ID: #

Blame Rev:

Test Plan: Revert Plan:

Differential Revision: https://reviews.facebook.net/D3147

db/db_bench.cc

@@ -252,7 +252,8 @@ class Stats {
       extra = rate;
     }
     AppendWithSpace(&extra, message_);
-    double throughput = (double)done_/seconds_;
+    double elapsed = (finish_ - start_) * 1e-6;
+    double throughput = (double)done_/elapsed;
 
     fprintf(stdout, "%-12s : %11.3f micros/op %ld ops/sec;%s%s\n",
             name.ToString().c_str(),