allow reused containers to linger

[StevenVanAcker]

containers can be reused so that the next incoming connection is forwarded to an already running container. When all connections are terminated, the container is also terminated. This means a new incoming connection incurs the overhead of having a new container launched. If containers are allowed to linger for a while (say 30 minutes) after the last connection has terminated, then any new connections will be put through faster.