Fix the memory leak in table index

Summary:

BinarySearchIndex didn't use unique_ptr to guard the block object nor
delete it in destructor, leading to valgrind failure for "definite
memory leak".

Test Plan:
re-ran the failed valgrind test cases

table/block_based_table_reader.cc

@@ -181,7 +181,7 @@ class BinarySearchIndexReader : public IndexReader {
       : IndexReader(comparator), index_block_(index_block) {
     assert(index_block_ != nullptr);
   }
-  Block* index_block_;
+  std::unique_ptr<Block> index_block_;
 };
 
 // TODO(kailiu) This class is only a stub for now. And the comment below is also