[bugfix]fix some bug of DMI mapper of modbus

[RyanZhaoXB]

fix some bug of DMI mapper of modbus:

1. fix bug for nil pointer of twin and context thread block
2. do not report if the twin data is not changed to prevent triggering traffic limiting
3. update log level and support twin for string, bool and float type

[kubeedge-bot]

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[kubeedge-bot]

@RyanZhaoXB: PR needs rebase.

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