fix(apisix-standalone): handle inline upstream update and delete

[bzp2010]

Description

I implemented inline upstream splitting in #319, but there seem to be some issues with the update and delete logic, specifically:

1. The split upstream may not be updated correctly.
2. When only the inline upstream is updated in the service, the service conf version is incorrectly increased.

This PR thoroughly reorganized this section and optimized the code.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible