Add connection window debug information.

This likely belongs in Netty somewhere, but I'm adding it here so I don't forget. It would be super useful to be able to quickly look at the connection level window and stream window, likely as some sort of debug log output.

Begin by locating the Netty-related handling for HTTP/2 connection-level and stream-level windows. Determine how debug logging is configured and what window values should be exposed; done means both window levels can be inspected through useful debug output.

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