network: make request_process_address() and friends take Link and corresponding object

network: make request_process_address() and friends take Link and corresponding object

This also renames e.g. request_process_address() -> address_process_request().

Also, this drops type checks such as `assert(req->type == REQUEST_TYPE_ADDRESS)`,
as in the later commits, the function of processing request, e.g.
`address_process_request()`, will be assigned to the Request object when
it is created. And the request type will be used to distinguish and to
avoid deduplicating requests which do not have any assigned objects,
like REQUEST_TYPE_DHCP4_CLIENT. Hence, the type checks in process functions
are mostly not necessary and redundant.

This is mostly cleanups and preparation for later commits, and should
not change any behavior.