Fix count/avg push down (#2445)

[ti-chi-bot]

This is an automated cherry-pick of #2445

What problem does this PR solve?

close #2313
#1840
close #1142

What is changed and how it works?

this bug is simply fixed by push down Constant(1) when count(1) or count(*)

• push down count by add pattern match for count, but
  • count(set) will not be pushed down because tikv can't accpect set type.
  • count(col1,col2,...) will not be pushed down because TiKV can't accpect (TIDB not support it)
• push down Constant(1) when count(1) or count(*)
• rewrite test and add test for count/avg

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• shiyuhang0
• xuanyu66

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[shiyuhang0]

/run-all-tests

[shiyuhang0]

/run-all-tests

[shiyuhang0]

/run-all-tests

[xuanyu66]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: e80c744