lightning: better estimate of the max-open-files

[glorv]

What problem does this PR solve?

Make better estimate of the max-open-files by also take import routines into account.

Close #1289

What is changed and how it works?

Check List

Tests

• Unit test

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release note

• No release note

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• 3pointer
• kennytm

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.

[kennytm]

merged into pingcap/tidb#27597 already.

[glorv]

/merge

[ti-chi-bot]

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

Commit hash: 8afaceb

[glorv]

/run-all-tests

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #1425.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #1426.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #1427.