EtcdWorker: fix EtcdWorker snapshot isolation (#2510)

[ti-chi-bot]

This is an automated cherry-pick of #2510

What problem does this PR solve?

• EtcdWorker's snapshot isolation is violated if there are two concurrent transactions one deleting and the other putting to the same key.
• EtcdWorker's snapshot isolation is violated if while A is running and trying to create a key k, B and C creates and deletes k sequentially.

What is changed and how it works?

• For the first case, we use equality of mod_revision to check for write-set intersection instead of inequality.
• For the second case, we use a global deleteCounter as a key to make deletions impossible to race with other operations.

Check List

Tests

• Unit test
• Integration test

Code changes

• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity

Related changes

• Need to cherry-pick to the release branch

Release note

Fix a bug in metadata management

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has not been approved.

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[amyangfei]

/merge

[ti-chi-bot]

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

Commit hash: f78b4a0

[amyangfei]

/run-integration-tests

[amyangfei]

/run-unit-tests