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

table/table.cc

@@ -196,7 +196,7 @@ Iterator* Table::BlockReader(void* arg,
                              bool* didIO) {
   Table* table = reinterpret_cast<Table*>(arg);
   Cache* block_cache = table->rep_->options.block_cache.get();
-  Statistics* const statistics = table->rep_->options.statistics;
+  std::shared_ptr<Statistics> statistics = table->rep_->options.statistics;
   Block* block = nullptr;
   Cache::Handle* cache_handle = nullptr;