network/queue: introduce RemoveRequest and relevant functions

network/queue: introduce RemoveRequest and relevant functions

This is similar to Request, but will be used on removing configuration
(e.g. address, route, and so on).

By using another queue for removing configuration, then we can avoid to
fill the reply callback buffer in sd-netlink by remove message calls.

Follow-up for 4e6a35e2b2fad0f167a71b63525f4210bc858bc6.