pkg: remove txn lock

[rleungx]

What problem does this PR solve?

Issue Number: Ref #5895.

What is changed and how does it work?

The lock here is used for multiple callers using the same Txn. But we barely have this situation in PD. At least, we don't have a such scenario for now. So this PR is going to remove it to improve the TSO saving performance.

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JmPotato
• binshi-bing

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[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.06 ⚠️

Comparison is base (791106f) 74.85% compared to head (a70b2dd) 74.79%.

❗ Current head a70b2dd differs from pull request most recent head f304c58. Consider uploading reports for the commit f304c58 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6208      +/-   ##
==========================================
- Coverage   74.85%   74.79%   -0.06%     
==========================================
  Files         395      395              
  Lines       38714    38706       -8     
==========================================
- Hits        28980    28952      -28     
- Misses       7208     7222      +14     
- Partials     2526     2532       +6     

Flag	Coverage Δ
unittests	74.79% <ø> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/storage/kv/etcd_kv.go	79.50% <ø> (+1.81%)	⬆️

... and 23 files with indirect coverage changes

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[binshi-bing]

It makes sense to me.

[binshi-bing]

It seems that we just simply removed the lock. Does it mean we guarantee that no etcdTxn crosses multiple goroutines? It makes sense to me.

[rleungx]

For now, it is.

[rleungx]

/merge

[ti-chi-bot]

@rleungx: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

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

Commit hash: a70b2dd

[ti-chi-bot]

@rleungx: Your PR was out of date, I have automatically updated it for you.

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