Support http pipelining (e.g. for apt)

[tkluck]

This is the likely root cause of #13 and #32 .

We can also "support" this by

• not forwarding subsequent requests upstream, and
• actively closing the downstream connection after the first response is fully forwarded

This will hopefully cause most clients to retry. (After all, it is established that pipelining has poor support: https://en.wikipedia.org/wiki/HTTP_pipelining.)