Replace short field names with more descriptive ones.

[matejrisek]

The decision to change the name of these fields came from reading the code further down in the scope and not being clear what c.c means.

This also aligns it with the convention from other structs in the same file like e.g. the session struct:

type session struct {
	auth     AuthMethod
	client   *http.Client
	endpoint *transport.Endpoint
	advRefs  *packp.AdvRefs
}

[pjbgf]

Thanks @matejrisek! 🙇