statistics: enlarge capacity of ddlEventCh to 1000

[xuyifangreeneyes]

What problem does this PR solve?

Issue Number: ref #38189 ref #36004

Problem Summary:

What is changed and how it works?

Enlarge the capacity of ddlEventCh to 1000, which can reduce the change of [ddl] fail to notify DDL event.

Check List

Tests

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

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Yisaer
• hawkingrei

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.

[hawkingrei]

/merge

[ti-chi-bot]

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

Commit hash: 57f2762

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #39562.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #39563.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #39564.

[sre-bot]

TiDB MergeCI notify

✅ Well Done! New fixed [2] after this pr merged.

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci/integration-cdc-test	🔴 failed 1, success 39, total 40	23 min	Existing failure
idc-jenkins-ci-tidb/common-test	🔴 failed 1, success 10, total 11	12 min	Existing failure
idc-jenkins-ci-tidb/sqllogic-test-1	✅ all 26 tests passed	6 min 19 sec	Fixed
idc-jenkins-ci-tidb/integration-ddl-test	✅ all 6 tests passed	5 min 3 sec	Fixed
idc-jenkins-ci-tidb/integration-common-test	🟢 all 17 tests passed	16 min	Existing passed
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	5 min 49 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	5 min 19 sec	Existing passed
idc-jenkins-ci-tidb/mybatis-test	🟢 all 1 tests passed	4 min 51 sec	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	2 min 50 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed