network: re-design request queue

This makes Request object takes hash, compare, free, and process functions.

With this change, the logic in networkd-queue.c can be mostly
independent of the type of the request or the object (e.g. Address) assigned
to the request, and it becomes simpler.

src/network/netdev/netdev.h

@@ -109,7 +109,6 @@ typedef enum NetDevCreateType {
 
 typedef struct Manager Manager;
 typedef struct Condition Condition;
-typedef struct Request Request;
 
 typedef struct NetDev {
         Manager *manager;
@@ -209,8 +208,6 @@ int netdev_set_ifindex(NetDev *netdev, sd_netlink_message *newlink);
 int netdev_generate_hw_addr(NetDev *netdev, Link *link, const char *name,
                             const struct hw_addr_data *hw_addr, struct hw_addr_data *ret);
 
-int independent_netdev_process_request(Request *req, Link *link, void *userdata);
-int stacked_netdev_process_request(Request *req, Link *link, void *userdata);
 int link_request_stacked_netdev(Link *link, NetDev *netdev);
 
 const char *netdev_kind_to_string(NetDevKind d) _const_;