Merge branch 'master' into performance

Conflicts:
	Makefile
	db/db_impl.cc
	db/db_test.cc
	db/memtable_list.cc
	db/memtable_list.h
	table/block_based_table_reader.cc
	table/table_test.cc
	util/cache.cc
	util/coding.cc

util/cache.cc

@@ -417,6 +417,7 @@ class ShardedLRUCache : public Cache {
   virtual size_t GetCapacity() const {
     return capacity_;
   }
+
   virtual size_t GetUsage() const {
     // We will not lock the cache when getting the usage from shards.
     // for (size_t i = 0; i < num_shard_bits_; ++i)
@@ -427,6 +428,10 @@ class ShardedLRUCache : public Cache {
     }
     return usage;
   }
+
+  virtual void DisownData() {
+    shards_ = nullptr;
+  }
 };
 
 }  // end anonymous namespace