conntrack: Periodically delete expired connections.

conntrack: Periodically delete expired connections.

This commit adds a thread that periodically removes expired connections.

The expiration time of a connection can be expressed by:

expiration = now + timeout

For each possible 'timeout' value (there aren't many) we keep a list.
When the expiration is updated, we move the connection to the back of the
corresponding 'timeout' list. This ways, the list is always ordered by
'expiration'.

When the cleanup thread iterates through the lists for expired
connections, it can stop at the first non expired connection.

Suggested-by: Joe Stringer <joe@ovn.org>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Joe Stringer <joe@ovn.org>