Implement timeout for keep-alive requests

The approach here should be:

• Use an AtomicInteger to track active requests in PlayDefaultUpstreamHandler, incrementing it for each new request, decrement it when the request is finished.
• Whenever the timeout reaches 0, and the connection is still open, set the timeout
• Any time a request is received, cancel the timeout
• When the timeout is triggered, close the connection

Let's make this default to 5 minutes, setting to -1 disables the timeout.

Start by locating PlayDefaultUpstreamHandler and tracing how requests begin and finish, along with the existing keep-alive handling. Implement the described active-request tracking and configurable five-minute timeout, with -1 disabling it; done means idle open connections close after the timeout while active or newly received requests cancel it.