[http_server] Simplify DRT node and table

[cong1920]

When I see the std::vecotr<std::shared_ptr<drt_node>> and std::vector<std::shared_ptr<std::string>> as the drt_node and dynamic_routing_table's members respectively, I cannot help wondering why the shared_ptr<T> are needed here. Later I realized the instance of the DRT table is allowed to copy therefore we need to keep its objects being shared across all instances. Somehow I still think it is a waste to have shared_ptr<T> manage each small object.

This PR is to have:

• unordered_set<string> to keep the strings other than std::vector<std::shared_ptr<std::string>>. unordered_set<string> is more straightforward and able to get of redundant duplicates automatically.
• drt_node_pool wrapping the vector<unique_ptr<drt_node>> and its instance is shared across all instances of drt_node to manage the lifetime of new drt_node instances.
• dynamic_routing_table is the only place to hold one shared_ptr<dynamic_routing_table_impl> and the dynamic_routing_table_impl is the previous implementation. therefore we only have one shared_ptr to manage one big object.

Will post this PR as draft to let @matt-42 have a review first. Will squash to one single commit before final merge.