Start an asynchronous send on a connected socket.

template<
    typename ConstBufferSequence,
    typename WriteHandler>
void async_send(
    const ConstBufferSequence & buffers,
    WriteHandler handler);
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template<
    typename ConstBufferSequence,
    typename WriteHandler>
void async_send(
    const ConstBufferSequence & buffers,
    socket_base::message_flags flags,
    WriteHandler handler);
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