Bugfix/issue 425 multishard incorrect result on distinct (#784)

* fix bug in distinct function when using multi shard(append-only)

* add smoek test

* fix test

* fix multishard incorrect distinct result

* fix logic error in distinct aggregate function

* revise code

* revise code second

* revise test cases

* add examples and notes

* fix format

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Co-authored-by: lizhou1111 <[email protected]>
Co-authored-by: Jasmine-ge <[email protected]>

Author: 3 people