Make DBIter class final

Summary:
DBIter is referenced in ArenaWrappedDBIter, which is a simple wrapper. If DBIter is final, some virtual function call can be avoided. Some functions can even be inlined, like DBIter.value() to ArenaWrappedDBIter.value() and DBIter.key() to ArenaWrappedDBIter.key(). The performance gain is hard to measure. I just ran the memory-only benchmark for readseq and saw it didn't regress. There shouldn't be any harm doing it. Just give compiler more choices.
Closes https://github.com/facebook/rocksdb/pull/2859

Differential Revision: D5799888

Pulled By: siying

fbshipit-source-id: 829788f91310c40282dcfb7e412e6ef489931143

db/db_iter.cc

@@ -50,7 +50,7 @@ static void DumpInternalIter(Iterator* iter) {
 // combines multiple entries for the same userkey found in the DB
 // representation into a single entry while accounting for sequence
 // numbers, deletion markers, overwrites, etc.
-class DBIter: public Iterator {
+class DBIter final: public Iterator {
  public:
   // The following is grossly complicated. TODO: clean it up
   // Which direction is the iterator currently moving?