lldpd: limit the maximum of neighbors per port and per protocol

lldpd memory can be exhausted by adding too many neighbors. Moreover,
the communication with lldpcli is limited to a fixed number of
neighbors (packets may not exceed 1 << 19 in size). Therefore, we put
a hard limit to the number of neighbors accepted for a given port, for
a given protocol.

The limit will be tunable in the future.

Feature suggested by Chris Yang. Closes #32.
lldpd: enforce TTL on remote chassis

Remote chassis were not expired in a timely manner. They were only
expired when a change happened on an interface. To fix this, we
maintain a timer that will be triggered when an interface is about to
expire. The timer is updated when a change happens and rescheduled
once it is run.