Delete session asynchronously - #1117

Purpose of the change

Improve user experience when deleting a session.

Description

When deleting a session, MemMachine will delete all the data from DB, which may take a long time if there are large number of memory entries. This can cause bad user experience or front-end timeout. In this PR, the session deletion is performed asynchronously. When the user requests delete, the session status is updated, and a background job does the actual deletion job. The side effect is that the same session ID can not be used before the actual background job is completed.

Fixes/Closes

Fixes #(issue number)

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (does not change functionality, e.g., code style improvements, linting)
• Documentation update
• Project Maintenance (updates to build scripts, CI, etc., that do not affect the main project)
• Security (improves security without changing functionality)

How Has This Been Tested?

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• Unit Test
• Integration Test
• End-to-end Test
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• Manual verification (list step-by-step instructions)

Test Results: [Attach logs, screenshots, or relevant output]

Checklist

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• I have signed the commit(s) within this pull request
• My code follows the style guidelines of this project (See STYLE_GUIDE.md)
• I have performed a self-review of my own code
• I have commented my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

Maintainer Checklist

• Confirmed all checks passed
• Contributor has signed the commit(s)
• Reviewed the code
• Run, Tested, and Verified the change(s) work as expected

Screenshots/Gifs

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Further comments

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