MDEV-8073 - Build error in sql/mdl.cc on OS X 10.10

C++03 seem to require default constructor for const objects even though they're
fully initialized. There is defect report for this:
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#253

Apparently some compilers (e.g. gcc) addressed this defect report, while some
(e.g. clang) did not.

Added empty default constructor to MDL_scoped_lock and MDL_object_lock classes.

Also replaced lf_hash_search_using_hash_value() with lf_hash_search(). The
latter will call mdl_key->hash_value() anyway.

sql/mdl.cc

@@ -346,6 +346,7 @@ class MDL_lock
   */
   struct MDL_scoped_lock : public MDL_lock_strategy
   {
+    MDL_scoped_lock() {}
     virtual const bitmap_t *incompatible_granted_types_bitmap() const
     { return m_granted_incompatible; }
     virtual const bitmap_t *incompatible_waiting_types_bitmap() const
@@ -382,6 +383,7 @@ class MDL_lock
   */
   struct MDL_object_lock : public MDL_lock_strategy
   {
+    MDL_object_lock() {}
     virtual const bitmap_t *incompatible_granted_types_bitmap() const
     { return m_granted_incompatible; }
     virtual const bitmap_t *incompatible_waiting_types_bitmap() const
@@ -741,8 +743,8 @@ MDL_lock* MDL_map::find_or_insert(LF_PINS *pins, const MDL_key *mdl_key)
   }
 
 retry:
-  while (!(lock= (MDL_lock*) lf_hash_search_using_hash_value(&m_locks, pins,
-                   mdl_key->hash_value(), mdl_key->ptr(), mdl_key->length())))
+  while (!(lock= (MDL_lock*) lf_hash_search(&m_locks, pins, mdl_key->ptr(),
+                                            mdl_key->length())))
     if (lf_hash_insert(&m_locks, pins, (uchar*) mdl_key) == -1)
       return NULL;
 
@@ -780,10 +782,8 @@ MDL_map::get_lock_owner(LF_PINS *pins, const MDL_key *mdl_key)
   }
   else
   {
-    lock= (MDL_lock*) lf_hash_search_using_hash_value(&m_locks, pins,
-                                                      mdl_key->hash_value(),
-                                                      mdl_key->ptr(),
-                                                      mdl_key->length());
+    lock= (MDL_lock*) lf_hash_search(&m_locks, pins, mdl_key->ptr(),
+                                     mdl_key->length());
     if (lock)
     {
       /*