Print complete statistics in db_stress

Summary: db_stress should alos print complete statistics like db_bench. Needed this when I wanted to measure number of delete-IOs dropped due to CheckKeyMayExist to be introduced to rocksdb codebase later- to make deltes in rocksdb faster

Test Plan: make db_stress;./db_stress --max_key=100 --ops_per_thread=1000 --statistics=1

Reviewers: sheki, dhruba, vamsi, haobo

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D11655

tools/db_stress.cc

@@ -994,10 +994,7 @@ class StressTest {
 
   void PrintStatistics() {
     if (dbstats) {
-      fprintf(stdout, "File opened:%ld closed:%ld errors:%ld\n",
-              dbstats->getTickerCount(NO_FILE_OPENS),
-              dbstats->getTickerCount(NO_FILE_CLOSES),
-              dbstats->getTickerCount(NO_FILE_ERRORS));
+      fprintf(stdout, "STATISTICS:\n%s\n", dbstats->ToString().c_str());
     }
   }