netfilter: ctnetlink: add timeout and protoinfo to destroy events

DESTROY events do not include the remaining timeout.

Add the timeout if the entry was removed explicitly. This can happen
when a conntrack gets deleted prematurely, e.g. due to a tcp reset,
module removal, netdev notifier (nat/masquerade device went down),
ctnetlink and so on.

Add the protocol state too for the destroy message to check for abnormal
state on connection termination.

Joint work with Pablo.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>