db: implement mutation_partition assignment operators

mutation_partition.cc

@@ -19,6 +19,13 @@ mutation_partition::~mutation_partition() {
     _row_tombstones.clear_and_dispose(std::default_delete<row_tombstones_entry>());
 }
 
+mutation_partition&
+mutation_partition::operator=(const mutation_partition& x) {
+    mutation_partition n(x);
+    std::swap(*this, n);
+    return *this;
+}
+
 void
 mutation_partition::apply(schema_ptr schema, const mutation_partition& p) {
     _tombstone.apply(p._tombstone);

mutation_partition.hh

@@ -170,6 +170,8 @@ public:
     mutation_partition(mutation_partition&&) = default;
     mutation_partition(const mutation_partition&);
     ~mutation_partition();
+    mutation_partition& operator=(const mutation_partition& x);
+    mutation_partition& operator=(mutation_partition&& x) = default;
     tombstone partition_tombstone() const { return _tombstone; }
     void apply(tombstone t) { _tombstone.apply(t); }
     void apply_delete(schema_ptr schema, const exploded_clustering_prefix& prefix, tombstone t);