domain, executor: make flush privilege propagate via etcd

[morgo]

What problem does this PR solve?

Issue Number: close #14914

Problem Summary:

Flush Privileges did not call NotifyUpdatePrivilege(), so peer tidb-servers were not notified.

What is changed and how it works?

What's Changed:

FLUSH PRIVILEGES now propagates to other tidb servers.

Check List

Tests

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

Because etcd notifications is not tested by the test-suite, I manually tested:

• On server tidb-4000 I created a user u1

• On server tidb-4001 I logged in as u1

• On server tidb-4000 I manually edited mysql.user so u1 had higher privileges. I then executed FLUSH PRIVILEGES.

• On server tidb-4001 I observed the changes were immediately visible.

• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

`FLUSH PRIVILEGES` now immediately propagates to other tidb-servers.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• tiancaiamao
• xhebox

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[tiancaiamao]

/merge

[ti-chi-bot]

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

Commit hash: ab3ee5b