Check current instance status before update.

[OlgaMaciaszek]

This is related to spring-cloud/spring-cloud-netflix#4094 as an attempted fix.

[OlgaMaciaszek]

@howardyuan could you please take a look?

[OlgaMaciaszek]

Adding pic with comments from offline discussion:

[OlgaMaciaszek]

Hi @howardyuan, regarding your comments before, I still use the pre-existing method under the hood from here, so setIsDirty will be called as well if we do change the status. I have added an exception when we verify the previous status first, as in this scenario we assume that it is the status that should be there in a correct workflow and if it's not set then possibly there was a race condition. null is returned in the pre-existing implementation if the status we want to change to is also the one already set - so nothing needs to be done. Seems like a different scenario to me. Let me know what you think. CC @spencergibb