Fixed memory leak in ShardedLRUCache

Summary: `~ShardedLRUCache()` was empty despite `init()` allocating memory on the heap. Fixed the leak by freeing memory allocated by `init()`.

Test Plan:
make check

Ran valgrind on db_test before and after patch and saw leaked memory went down

Reviewers: vamsi, dhruba, emayanke, sheki

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D7791

util/cache.cc

@@ -304,7 +304,9 @@ class ShardedLRUCache : public Cache {
      : last_id_(0) {
     init(capacity, numShardBits);
   }
-  virtual ~ShardedLRUCache() { }
+  virtual ~ShardedLRUCache() {
+    delete[] shard_;
+  }
   virtual Handle* Insert(const Slice& key, void* value, size_t charge,
                          void (*deleter)(const Slice& key, void* value)) {
     const uint32_t hash = HashSlice(key);