Fix addSessionCount() in multu-thread case

[Aiee]

What type of PR is this?

• bug
• feature
• enhancement

What problem(s) does this PR solve?

Issue(s) number:

Close #5392

Description:

When multiple threads try to increment session counts at the same time, there is a chance that emplace() will return false. To avoid this issue, it's recommended to use insert_or_assign instead of emplace because the check for session counts doesn't need to be too strict.

How do you solve it?

Use insert_or_assign instead of emplace.

Special notes for your reviewer, ex. impact of this fix, design document, etc:

Checklist:

Tests:

• Unit test(positive and negative cases)
• Function test
• Performance test
• N/A

Affects:

• Documentation affected (Please add the label if documentation needs to be modified.)
• Incompatibility (If it breaks the compatibility, please describe it and add the label.）
• If it's needed to cherry-pick (If cherry-pick to some branches is required, please label the destination version(s).)
• Performance impacted: Consumes more CPU/Memory

Release notes:

Please confirm whether to be reflected in release notes and how to describe:

ex. Fixed the bug .....

[yixinglu]

LGTM