mutation: Set cell using clustering_key_prefix

Change the clustering key argument in mutation::set_cell from
exploded_clustering_prefix to clustering_key_prefix, which allows for
some overall code simplification and fewer copies. This mostly affects
the cql3 layer.

Signed-off-by: Duarte Nunes <duarte@scylladb.com>

keys.hh

@@ -254,6 +254,9 @@ public:
         size_t operator()(const TopLevel& o) const {
             return _t->hash(o);
         }
+        size_t operator()(const TopLevelView& o) const {
+            return _t->hash(o.representation());
+        }
     };
 
     struct equality {