abstract_tcp_server2: fix send queue limit warning spam
abstract_tcp_server2: fix send queue limit warning spam

When the send queue limit is reached, it is likely to not drain
any time soon. If we call close on the connection, it will stay
alive, waiting for the queue to drain before actually closing,
and will hit that check again and again. Since the queue size
limit is the reason we're closing in the first place, we call
shutdown directly.

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