@Override
    public long readFixed64() throws IOException {
        checkIfPackedField();
        return readRawLittleEndian64();
    }
Read a {@code fixed64} field value from the internal buffer.
jraft-rheakv/rheakv-core/src/main/java/com/alipay/sofa/jraft/rhea/serialization/impl/protostuff/io/NioBufInput.java