fix recreating users' default tenaunt relations when loading user

[bowenliang123]

• fix bugs in load_user introduced by #2341
• 1. skip auto creating default tenant in this method. for the reasons that
  • 1.1 it causes leak as it does not acquire locks to guarantee only one default tenant will be created
  • 1.2 it's not a right timing to create user's default tenant. A better way is to do it in register service.
• 2. reuse existed available tenant for the user

Caused by 1.2.

[bowenliang123]

cc @crazywoola @takatost

[takatost]

We need to create a personal tenant as the owner when invited to another tenant. We need to make this logic before we can merge together.