The fields in the neighbor variant should have a defined order.

Instead of sorting the hash table entries while constructing the
variant in lldp_neighbor_to_variant(), refactor the management of
the TLV attributes.
As we only support known attributes, we can
store them in an array at a known index instead of putting them
in a hash table.
An alternative would be to have explict fields for every known
attribute. That would be even more efficient, but requires more
work when adding new attributes.