*: replace pessimistic default config option with global variable

[coocood]

What problem does this PR solve?

The 'PessimisticTxn.Default' option in the Config requires a server restart to change.
It can be inconsistent for multiple servers.

What is changed and how it works?

• Add global scope for tidb_txn_mode system variable.
• Remove PessimisticTxn.Default option in the config.
• Change the default value of PessimisticTxn.Enable to true.

Check List

Tests

• Unit test

Code changes

• Has exported variable/fields change

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to update the tidb-ansible repository

Release note

• Add global scope for tidb_txn_mode system variable, set enable option to 'true' and remove default option in the [pessimistic-txn] section of the config file.

[codecov]

Codecov Report

Merging #12041 into master will decrease coverage by 0.1206%.
The diff coverage is 100%.

@@               Coverage Diff                @@
##             master     #12041        +/-   ##
================================================
- Coverage   81.4711%   81.3505%   -0.1207%     
================================================
  Files           447        447                
  Lines         95759      95649       -110     
================================================
- Hits          78016      77811       -205     
- Misses        12181      12303       +122     
+ Partials       5562       5535        -27

[jackysp]

LGTM

[tiancaiamao]

LGTM

[sre-bot]

/run-all-tests

[sre-bot]

@coocood merge failed.

[coocood]

/run-unit-test

[sre-bot]

cherry pick to release-3.0 failed