The commit fixes BIND hanging when browsers end HTTP/2 streams
prematurely (for example, by sending RST_STREAM). It ensures that
isc__nmsocket_prep_destroy() will be called for an HTTP/2 stream,
allowing it to be properly disposed.
The problem was impossible to reproduce using dig or DoH benchmarking
software (e.g. flamethrower) because these do not tend to end HTTP/2
streams prematurely.
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