cmd/importer: support probability in building incremental column

[eurekaka]

What problem does this PR solve?

Generate column values in incremental order mostly, but with some exceptions controlled by input parameter.

What is changed and how it works?

Support new rule probability=xxx which should be used together with incremental, to control the exceptions in overall incremental order.

Check List

Tests

• Manual test (add detailed scripts or steps below)

run script:
./bin/importer -t "create table t(a date comment '[[incremental=1;repeats=3;step=-1;probability=80]]');" -c 1 -n 10 -P 4000

check table rows:
mysql> select * from t;
+------------+
| a          |
+------------+
| 2019-05-14 |
| 2019-05-02 |
| 2019-05-13 |
| 2019-06-05 |
| 2019-05-13 |
| 2019-05-12 |
| 2019-05-12 |
| 2019-08-26 |
| 2019-05-11 |
| 2019-05-11 |
+------------+
10 rows in set (0.00 sec)

Code changes

N/A

Side effects

N/A

Related changes

N/A

[codecov]

Codecov Report

Merging #10454 into master will decrease coverage by 0.0043%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##             master     #10454        +/-   ##
================================================
- Coverage   77.3086%   77.3043%   -0.0044%     
================================================
  Files           412        412                
  Lines         86575      86629        +54     
================================================
+ Hits          66930      66968        +38     
- Misses        14503      14512         +9     
- Partials       5142       5149         +7

[zz-jason]

should we specially handle 0 step?

[eurekaka]

If step is 0, we can use range or set rule, instead of incremental?

[zz-jason]

@lamxTyler How do you think?

[alivxxx]

0 step also makes senses, for example, we only want constant data.

[eurekaka]

Yes, but if we want constant data, we don't need to use incremental, a better way is using set?

[zz-jason]

I prefer to do this optimization in the future, for now, let's make it work.

[XuHuaiyu]

please IGNORE this comment
/run-all-tests tidb-test=pr/814

[XuHuaiyu]

/run-all-tests tidb-test=pr/814

[alivxxx]

LGTM

[zz-jason]

LGTM