make uuid_t hashable

include/db_lib/db_types.hpp

@@ -117,6 +117,17 @@ struct is_map_key_type_same_as_enum {
 
 };
 
+// hash operator to be able use uuid_t as a hashmap key
+template<>
+struct std::hash<db::uuid_t> {
+    std::size_t operator()(const db::uuid_t& k) const {
+        using std::size_t;
+        using std::hash;
+
+        return hash<size_t>()(hash<uint64_t>()(k.uuid_upper) ^ hash<uint64_t>()(k.uuid_lower));
+    }
+};
+
 
 /**
  * @brief Generate type conversion for enum class macros in the libpqxx library