bgpd: Handle Graceful-Restart capability with dynamic capability

bgpd: Handle Graceful-Restart capability with dynamic capability

Graceful-Restart restart time is exchanged using OPEN messages. In order to
reduce restart time before doing an actual graceful restart, it might be useful
to increase the time, but this is not possible without resetting the session.

With this change, it's possible to send dynamic capability with a new value, and
GR will respect a new reset time value.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>