Address MS Visual Studio 2017 issue with autovector

Summary:
This addresses https://github.com/facebook/rocksdb/issues/2262
Closes https://github.com/facebook/rocksdb/pull/2333

Differential Revision: D5097941

Pulled By: siying

fbshipit-source-id: fb33582bfe7883ecc3f6da028703982522b5f75f

util/autovector.h

@@ -98,16 +98,16 @@ class autovector {
       return old;
     }
 
-    self_type operator-(difference_type len) {
+    self_type operator-(difference_type len) const {
       return self_type(vect_, index_ - len);
     }
 
-    difference_type operator-(const self_type& other) {
+    difference_type operator-(const self_type& other) const {
       assert(vect_ == other.vect_);
       return index_ - other.index_;
     }
 
-    self_type operator+(difference_type len) {
+    self_type operator+(difference_type len) const {
       return self_type(vect_, index_ + len);
     }
 
@@ -126,11 +126,23 @@ class autovector {
       assert(vect_->size() >= index_);
       return (*vect_)[index_];
     }
+
+    const_reference operator*() const {
+      assert(vect_->size() >= index_);
+      return (*vect_)[index_];
+    }
+
     pointer operator->() {
       assert(vect_->size() >= index_);
       return &(*vect_)[index_];
     }
 
+    const_pointer operator->() const {
+      assert(vect_->size() >= index_);
+      return &(*vect_)[index_];
+    }
+
+
     // -- Logical Operators
     bool operator==(const self_type& other) const {
       assert(vect_ == other.vect_);