expression: refactor builtin function RAND to be compatible with MySQL

[XuHuaiyu]

What problem does this PR solve?

1. builtinRandSig/builtinRandWithSeedSig.randGen may cause data race when Project executing parallelly.
2. Results of rand(N) is not compatible with MySQL as expression: refactor builtin function RAND to be compatible with MySQL #9387 (comment) mentioned.

What is changed and how it works?

1. Eager evaluate seed for rand() in getFunction.
2. Check whether args[0] is a constant during getFunction:
   If so, return builtinRandSig rather than builtinRandWithSeedSig;
   If not, evaluate seed each time Rand invoked.

Check List

Tests

integration tests

Code changes

• Has exported function/method change
• Has exported variable/fields change

Side effects

N/A

Related changes

• Need to cherry-pick to the release branch

[XuHuaiyu]

/run-all-tests

[zz-jason]

can we make a copy for each parallel Project worker?

[codecov-io]

Codecov Report

Merging #9387 into master will increase coverage by <.01%.
The diff coverage is 58.82%.

@@            Coverage Diff             @@
##           master    #9387      +/-   ##
==========================================
+ Coverage   67.17%   67.17%   +<.01%     
==========================================
  Files         373      373              
  Lines       78292    78298       +6     
==========================================
+ Hits        52590    52599       +9     
+ Misses      20992    20987       -5     
- Partials     4710     4712       +2

Impacted Files	Coverage Δ
expression/builtin_math.go	70.16% <58.82%> (+0.63%)	⬆️
store/tikv/scan.go	73.94% <0%> (-3.37%)	⬇️
expression/schema.go	94.53% <0%> (+0.78%)	⬆️
infoschema/infoschema.go	77.63% <0%> (+1.31%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 79a88a0...b865280. Read the comment docs.

[XuHuaiyu]

@zz-jason
Yes, we can, thus we need to add a Clone function for *expression.EvaluatorSuite. I'll modify it.

[alivxxx]

LGTM

[zz-jason]

LGTM

[zz-jason]

/run-all-tests

[ciscoxll]

/run-all-tests

[zz-jason]

@XuHuaiyu Please cherry pick this commit to release 2.1 and release 2.0