Correctly cleanup HTTP/2 connection channel

[aryan-25]

Motivation:

The inbound HTTP/2 stream iterator can exit when the server task is cancelled, or when the stream multiplexer finishes or throws. However, when this happens, we currently do not close the connection channel. We should fix this.

Modifications:

• Updated the serveHTTP2Connection method to also close the connection channel when the inbound stream iteration exits.
• Added a regression test that checks the connection channel is closed when the server task is cancelled during an active connection. This test indefinitely stalls before this patch.

Result:

Active HTTP/2 connections are now cleaned up correctly when the inbound stream iteration exits.