lldpd: fast start support

This patch adds support for lldpd fast start.

The feature is currently disabled by default.
A cli command to enable the feature is in the
works.

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
lldpd: make fast start work only when receiving LLDP-MED LLDPDU
lldpd: enable fast start by default
lldpcli: allow to enable/disable fast-start and set delay