isisd, ospfd: IGPs detect LDP down via zapi client close message

isisd, ospfd: IGPs detect LDP down via zapi client close message

When ldp-sync is configured, IGPs take action if the LDP process goes down.

Currently, IGPs detect the LDP process is down if they do not receive a
periodic 'hello' message from LDP within 1 second.

Intermittently, this heartbeat mechanism causes false topotest failures.
When the failure occurs, LDP is busy receiving messages from zebra for a
few seconds.  During this time, LDP does not send the expected periodic
message.

With this change, IGPs detect LDP down via zapi client close message.

Signed-off-by: Karen Schoener <karen@voltanet.io>