fix issue #4816 some aggregate function work incorrectly in cluster mode

[mengjinglei]

• CHANGELOG.md updated
• Rebased/mergable
• Tests pass
• Sign CLA (if not already signed)

[otoolep]

Is there any way to unit test this?

[mengjinglei]

I noticed that there is no unit test case for InitializeUnmarshaller function in tsdb/functions_test.go, but I'd like to have a try.

[mengjinglei]

Unmarshaller unmarshal bytes receive from remote node to interface that reducer can use, for min and max functions, the input interface of reducer is expect to be minMaxMapOut, but there is no specific unmarshaller for min and max functions, so the bytes are unmarshal by default unmarshaller whose output can not be recognize by the reducer

[mengjinglei]

@otoolep

[otoolep]

+1

@dgnorton -- can you please also review?

[otoolep]

CLA confirmed signed.

[dgnorton]

+1

[mengjinglei]

@otoolep

[otoolep]

Thanks @mengjinglei