output perf_context in db_bench readrandom

Summary:
Add helper function to print perf context data in db_bench if enabled.
I didn't find any code that actually exports perf context data. Not sure
if I missed anything

Test Plan: ran db_bench

Reviewers: haobo, sdong, igor

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16575

include/rocksdb/perf_context.h

@@ -7,6 +7,7 @@
 #define STORAGE_ROCKSDB_INCLUDE_PERF_CONTEXT_H
 
 #include <stdint.h>
+#include <string>
 
 namespace rocksdb {
 
@@ -26,6 +27,8 @@ struct PerfContext {
 
   void Reset(); // reset all performance counters to zero
 
+  std::string ToString() const;
+
   uint64_t user_key_comparison_count; // total number of user key comparisons
   uint64_t block_cache_hit_count;     // total number of block cache hits
   uint64_t block_read_count;          // total number of block reads (with IO)