Fixes #7606.

Couple of recent changes worth noting here:

• Send GOAWAY to server on Client Transport Shutdown #7015 added logic to send a GOAWAY frame on client transport shutdown. Unfortunately the handler responsible for writing the GOAWAY frame was holding on to the client transport mutex when attempting to write the GOAWAY frame.
• transport: add timeout for writing GOAWAY on http2Client.Close() #7371 introduced a timeout in the client transport shutdown handler to wait for loopyWriter to exit (after enqueueing the above GOAWAY frame on the controlbuf). This was done to ensure that the client transport shutdown can complete in the face of a hanging network connection that blocks forever when attempting to write the above GOAWAY frame

Description of the deadlock:

• During client transport shutdown, after enqueueing the GOAWAY frame on the controlbuf, http2Client.Close calls http2Client.GetGoAwayReason to fetch the last GOAWAY's debug message, and the latter attempts to grab http2Client.mu.
• http2Client.outgoingGoAwayHandler holds http2Client.mu when it is attempting to write the GOAWAY frame. So, if the underlying network connection is hanging, this method will not release the mutex, and therefore http2Client.GetGoAwayReason will not be able to grab the same mutex, and thereby http2Client.Close will deadlock.

RELEASE NOTES:

• transport: prevent deadlock in client transport shutdown when writing the GOAWAY frame hangs.