log: generate digest and log it in slow log

[lysu]

What problem does this PR solve?

follow up to pingcap/parser#32, add digest to slow log

so we can awk or sql agg sql by digest to analysis query faster(even without pt-digest)

What is changed and how it works?

• for each query generate digest and cache it in stmtCtx(later sql bind or audit can use it directly)
• add slow log filed
• add slow log table field
• add admin show slow field(maybe we can do more use digest for show slow but maybe in following pr)

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Code changes

• Add stmt variable

Side effects

• Possible performance regression(but lexer is fast)

Related changes

• Need to cherry-pick to the release 2.1 when master work well

---

This change is 

[codecov]

Codecov Report

Merging #9662 into master will decrease coverage by 0.0118%.
The diff coverage is 76.4705%.

@@               Coverage Diff                @@
##             master      #9662        +/-   ##
================================================
- Coverage   67.4017%   67.3899%   -0.0119%     
================================================
  Files           377        377                
  Lines         79443      79457        +14     
================================================
  Hits          53546      53546                
- Misses        21117      21128        +11     
- Partials       4780       4783         +3

[jackysp]

/run-all-tests

[jackysp]

LGTM

[lysu]

/run-all-tests

[zz-jason]

LGTM

[lysu]

/run-all-tests

[lysu]

/run-common-test tidb-test=pr/755
/run-integration-common-test tidb-test=pr/755

[lysu]

/run-common-test tidb-test=pr/755
/run-integration-common-test tidb-test=pr/755

[lysu]

/run-all-tests tidb-test=pr/755