network: introduce reference counter for Request object

network: introduce reference counter for Request object

Currently, all Request object are always owned by Manager, and freed
when it is processed, especially, soon after a netlink message is sent.
So, it is not necessary to introduce the reference counter.

In a later commit, the Request object will _not_ be freed at the time
when a netlink message is sent, but assigned to the relevant netlink
slot as a userdata, and will be freed when a reply is received. So, the
owner of the Request object is changed in its lifetime. In that case, it
is convenient that the object has reference counter to avoid memleak or
double free.