bgpd: Streamline GR config, act on change immediately

bgpd: Streamline GR config, act on change immediately

Streamline the BGP graceful-restart configuration at the global and
peer level some more. Similar to many other neighbor capability
parameters like MP and ENHE, reset the session immediately upon a
change to the configuration. This will be more aligned with the
transactional UI model also and will not require a separate 'clear'
command to be executed.

Note: Peer-group graceful-restart configuration is not yet supported.

Signed-off-by: Vivek Venkatraman <vivek@nvidia.com>