bugfix: fix internalserverError while connecting mongoDB with clustering uri

[wu0h961738]

🔍 What type of PR is this?

bug

👀 What this PR does / why we need it:

• My pull request adheres to the code style of this project
• My code requires changes to the documentation
• I have updated the documentation as required
• All the tests have passed

🅰 Which issue(s) this PR fixes:

Fixes openimsdk/open-im-server#1627

📝 Special notes for your reviewer:

• root cause: context management in Golang.

  In the m.tx code block within the initialization function, misusing the ctx parameter as the parameter for end session and Transaction function calls actually captures a context that does not exist in the init function. Therefore, it causes the first user registration to succeed in a clustering architecture (because at this point the ctx is the same as the actual execution context of the function). However, the second execution fails (since the ctx within m.tx is garbage-collected at the end of the first task).

🎯 Describe how to verify it

With config of monogdb changed from address to clustering uri format (mongo+srv://XX), sending request multiple times to the API /user/user_register won't show the internal server error again.
While at the same time, the test of the origin scenario in which the standalone mongoDB serve as the database is still successful functioning as well. 

📑 Additional documentation e.g., RFC, notion, Google docs, usage docs, etc.: