Relay graceful close

[karalabe]

After the client requests a close, there should be a grace timeout in which no new requests are accepted, but already running ones should be allowed to complete to time out. Additionally, inbound ones should be either forwarded to the client, or re-routed to possible new clients.

Graceful tunnel termination should also be inspected, but that will require a larger redesign, so it will probably be done later. Just don't forget about it.