db: fix mutation::set_*_cell() applied twice to same column

With a collection, setting two separate elements in a collection would
cause the second to override the first.  This also applies, with much
smaller effect, to normal cells (for example, updating the same counter
twice, or issuing two updates to the same cell but with different timestamps,
via thrift).

Fix by merging the two values rather than replacing the old one.

database.hh

@@ -95,26 +95,27 @@ public:
     mutation(const mutation&) = default;
 
     void set_static_cell(const column_definition& def, atomic_cell_or_collection value) {
-        emplace_or_insert(p.static_row(), def.id, std::move(value));
+        update_column(p.static_row(), def, std::move(value));
     }
 
     void set_clustered_cell(const clustering_prefix& prefix, const column_definition& def, atomic_cell_or_collection value) {
         auto& row = p.clustered_row(serialize_value(*schema->clustering_key_type, prefix));
-        emplace_or_insert(row, def.id, std::move(value));
+        update_column(row, def, std::move(value));
     }
 
     void set_clustered_cell(const clustering_key& key, const column_definition& def, atomic_cell_or_collection value) {
         auto& row = p.clustered_row(key);
-        emplace_or_insert(row, def.id, std::move(value));
+        update_column(row, def, std::move(value));
     }
 private:
-    static void emplace_or_insert(row& row, column_id id, atomic_cell_or_collection&& value) {
+    static void update_column(row& row, const column_definition& def, atomic_cell_or_collection&& value) {
         // our mutations are not yet immutable
+        auto id = def.id;
         auto i = row.lower_bound(id);
         if (i == row.end() || i->first != id) {
             row.emplace_hint(i, id, std::move(value));
         } else {
-            i->second = std::move(value);
+            merge_column(def, i->second, value);
         }
     }
     friend std::ostream& operator<<(std::ostream& os, const mutation& m);