Remove deprecated use of std::iterator<>

As of C++17 std::iterator<> is deprecated, so update
MutationLog::iterator to use member types rather than inheriting from
std::iterator<>.

For more information:
https://www.fluentcpp.com/2018/05/08/std-iterator-deprecated/

Change-Id: I6ea891dd5ea406929ca8f88914bd22400df23d8b
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/172770
Tested-by: Build Bot <[email protected]>
Reviewed-by: Dave Rigby <[email protected]>

Author: rdemellow

engines/ep/src/mutation_log.h

@@ -306,9 +306,13 @@ class MutationLog {
      *
      * A ReadException may be thrown at any point along iteration.
      */
-    class iterator  : public std::iterator<std::input_iterator_tag,
-                                           const MutationLogEntry> {
+    class iterator {
     public:
+        using iterator_category = std::input_iterator_tag;
+        using value_type = const MutationLogEntry;
+        using difference_type = const MutationLogEntry;
+        using pointer = const MutationLogEntry*;
+        using reference = const MutationLogEntry&;
 
         iterator(const iterator& mit);