can: remove keepOpenOnPeerClosed flag from Http.Register command

[sirthias]

It doesn't make sense to be able to configure the keepOpenOnPeerClosed behavior for HTTP.

Here is how spray-can should behave:

Client side

The Tcp.Register command to the IO layer (in the HttpClientConnection) should have keepOpenOnPeerClosed = settings.sslEncryption.
Additionally, because the SslTlsSupport pipeline stage might be active but encryption still be disabled, we need to always actively close the connection whenever we see a PeerClosed event (this logic needs to go into the ClientFrontend stage).

Server side

On the server-side it might be that the client half-closes the connection before having received the response. So we always enable keepOpenOnPeerClosed.
When we see a PeerClosed event we can close the connection immediately if no response is currently pending. Otherwise we switch into a mode that causes the last pending response that goes out to have a Connection: close header (which causes the ResponseRendering stage to actively close the connection when the response has been dispatched to the IO layer.