prevent unnecessary go routines by limiting them when only the model is AfterFindable

[sio4]

What is being done in this PR?

• readability improvement for exclusive blocks (follow up of Add AfterEagerFind() #802)
• prevent unnecessary go routines by limiting them when only the model is AfterFindable (feature: improve performance if the element of the Value of *Model does not implement the AfterFindable interface. #799)

What are the main choices made to get to this solution?

• Deep cascaded if blocks are not beautiful but sometimes easy to read. For this case, the exclusive condition is used for two blocks one by one, so grouping them with one if-else will make it easy to read.
• Unnecessary go routine creation was reported as feature: improve performance if the element of the Value of *Model does not implement the AfterFindable interface. #799 but did not addressed yet. The issue is reasonable, and also it looks better to read.

closes #799