[go][model] Use pointers to avoid circular references

[wy-z]

PR checklist

• Read the contribution guildelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/go-petstore.sh and ./bin/security/go-petstore.sh if updating the go (e.g. php, ruby, python, etc) code generator or go client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

• Use pointers to avoid circular references
• Abandon omitempty tag when the field is required

[wing328]

@wy-z thanks for the PR but I think it should be filed against 2.3.0 as it's a breaking change.

Here is the related discussion: #2330

cc @neilotoole @guohuang @antihax

[wy-z]

@wing328 ok~
It seems that #4440 is better~