When the instance is not running (after creation or after stop), there
is no need to keep the GHashTable around.

Create it when needed (during start) and clear it during stop. This
makes it slightly cheaper to keep a NMLldpListener instance around,
if it's currently not running.

NMDevice already keeps the NMLldpListener around, even after stopping
it. It's not clear whether the instance will be started again, so also
clear the GHashTable. Also, one effect is that if you initially were in
a network with many LLDP neibors, after stop and start, the GHashTable
now gets recreated and may not need to allocate a large internal array
as before.