There is no public API to modify a NMLldpNeighbor instance. They are only
created by NMDevice and afterwards immutable. There is no point to duplicate
them, when we can just take a ref.
Note that this means we must be careful if we ever add API to mutate a
NMLldpNeighbor. But immutable types are awesome, so we probably should
never do that anyway.
At least ref-counting should be thread safe for our public API.
Note that NMLldpNeighbor has no public API for mutating a neighbor
instance. Especially for immutable types, it is very attractive that
ref-counting is thread-safe.
Also use slice allocator for NMLldpNeighbor structs.
