fixed concurrent map write and read for conn matrix map

[Wang]

1. Are you opening this pull request for bug-fixes, optimizations or new feature?

bug fix

2. Please describe how these code changes achieve your intention.

There is a small chance that a connection reading data and writing data at the same time triggers el.close, resulting in concurrent read and write the connMatrix map.

3. Please link to the relevant issues (if any).

fixes #632

4. Which documentation changes (if any) need to be made/updated because of this PR?

4. Checklist

• [ ] I have squashed all insignificant commits.
• [ ] I have commented my code for explaining package types, values, functions, and non-obvious lines.
• [ ] I have written unit tests and verified that all tests passes (if needed).
• [ ] I have documented feature info on the README (only when this PR is adding a new feature).
• [ ] (optional) I am willing to help maintain this change if there are issues with it later.

[panjf2000]

I appreciate your effort here, but this is an incorrect PR.