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.
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.
Yu Watanabe