lightning: better estimate of the max-open-files (#1414)

[ti-chi-bot]

This is an automated cherry-pick of #1414

What problem does this PR solve?

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

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:

• YuJuncen
• glorv

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[glorv]

/merge

[ti-chi-bot]

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

Commit hash: 4fac5fb