Fix NPE when failing to decode response from server

[jekh]

This PR should fix issue #303. It does two things:

• Keeps track of the current request in ProxyToServerConnection as soon as it is sent to the server. Removes the "queue" of current requests and the need to "pop" from the queue at some point while processing the response from the server. The code always assumed there would be only one request per connection, so having a stack of requests is unnecessary.
• When netty fails to decode a response from the server, this will "substitute" a new 502 Bad Gateway response and send that to the client, closing the connection to the server in the process. This is the most sensible thing we can do, since a decoder failure means the response is too malformed to make any sense of.

The current code already assumes there is one request outstanding per connection at a time. This is sensible in HTTP 1.1 (pipelining notwithstanding, though that's rarely used), but will need to change when we support HTTP/2.

[jekh]

I've been testing with this patch for the last several days and haven't seen any issues. Since it's been such a problem for a long time, I'll go ahead and merge. If anybody has any review comments/objections, I'll correct or revert as needed.