Make BlockIter final

Summary:
Now BlockBasedTableIterator directly uses BlockIter. By making BlockIter final, we can prevent unintended virtual function overriding.
Closes https://github.com/facebook/rocksdb/pull/3828

Differential Revision: D7933816

Pulled By: siying

fbshipit-source-id: 026a08cb5c5b6d3d6f44743152b4251da4756f2c

table/block.h

@@ -195,7 +195,7 @@ class Block {
   void operator=(const Block&) = delete;
 };
 
-class BlockIter : public InternalIterator {
+class BlockIter final : public InternalIterator {
  public:
   // Object created using this constructor will behave like an iterator
   // against an empty block. The state after the creation: Valid()=false