memory manage statistics

Summary:
Earlier Statistics object was a raw pointer. This meant the user had to clear up
the Statistics object after creating the database. In most use cases the database is created in a function and the statistics pointer is out of scope. Hence the statistics object would never be deleted.
Now Using a shared_ptr to manage this.

Want this in before the next release.

Test Plan: make all check.

Reviewers: dhruba, emayanke

Reviewed By: emayanke

CC: leveldb

Differential Revision: https://reviews.facebook.net/D9735

include/leveldb/options.h

@@ -11,6 +11,7 @@
 #include <vector>
 #include <stdint.h>
 #include "leveldb/slice.h"
+#include "leveldb/statistics.h"
 
 namespace leveldb {
 
@@ -20,7 +21,6 @@ class Env;
 class FilterPolicy;
 class Logger;
 class Snapshot;
-class Statistics;
 
 using std::shared_ptr;
 
@@ -258,7 +258,7 @@ struct Options {
   // If non-null, then we should collect metrics about database operations
   // Statistics objects should not be shared between DB instances as
   // it does not use any locks to prevent concurrent updates.
-  Statistics* statistics;
+  shared_ptr<Statistics> statistics;
 
   // If true, then the contents of data files are not synced
   // to stable storage. Their contents remain in the OS buffers till the

include/leveldb/statistics.h

@@ -124,10 +124,10 @@ class Statistics {
 };
 
 // Ease of Use functions
-inline void RecordTick(Statistics* const statistics,
+inline void RecordTick(std::shared_ptr<Statistics> statistics,
                        Tickers ticker,
                        uint64_t count = 1) {
-  if (statistics != nullptr) {
+  if (statistics) {
     statistics->recordTick(ticker, count);
   }
 }