abstract_tcp_server2: avoid deadlock waiting for send queue to drain
abstract_tcp_server2: avoid deadlock waiting for send queue to drain

If we reach the send queue size limit, we need to release the lock,
or we will deadlock and it will never drain.

If we reach that limit, it's likely there's another problem in the
first place though, so it will probably not drain in practice either,
unless some kind of transient network timeout.

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