MergingIterator uses autovector instead of vector

Summary:
Use autovector in MergingIterator so that if there are 4 or less child iterators in it, iterator wrappers are inline, which is more likely to be cache friendly.

Based on one test run with a shadow traffic of one product, it reduces CPU of MergingIterator::Seek() by half.

Test Plan: make all check

Reviewers: haobo, yhchiang, igor, dhruba

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18531

table/iterator_wrapper.h

@@ -20,6 +20,13 @@ class IteratorWrapper {
   explicit IteratorWrapper(Iterator* iter): iter_(nullptr) {
     Set(iter);
   }
+  IteratorWrapper(const IteratorWrapper&) {
+    // Iterator wrapper exclusively owns iter_ so it cannot be copied.
+    // Didn't delete the function because vector<IteratorWrapper> requires
+    // this function to compile.
+    assert(false);
+  }
+  void operator=(const IteratorWrapper&) = delete;
   ~IteratorWrapper() { delete iter_; }
   Iterator* iter() const { return iter_; }
 
@@ -35,7 +42,6 @@ class IteratorWrapper {
     }
   }
 
-
   // Iterator interface methods
   bool Valid() const        { return valid_; }
   Slice key() const         { assert(Valid()); return key_; }