clustering_row: Add new .apply() overload

The clustering_row is a wrapper over the deletable_row and
facilitates the apply-creation of the latter from some other
objects. Soon it will accept the deletable_row itself for
apply()-ing.

Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>

mutation_fragment.hh

@@ -97,6 +97,9 @@ public:
     void apply(const schema& s, const rows_entry& r) {
         _row.apply(s, deletable_row(s, r.row()));
     }
+    void apply(const schema& s, const deletable_row& r) {
+        _row.apply(s, deletable_row(s, r));
+    }
 
     position_in_partition_view position() const;