Only overwrite Posts that don't have local changes

[mzorz]

This PR does two things:

1. when retrieving the Posts list, only dispatch fetch action for posts that don't have local changes.
   2. also when listening to completed FETCH_ACTION, only overwrite Posts that don't have local changes updated as per convo in in Possible content loss when a draft is published from elsewhere WordPress-Android#5984 (comment)

Otherwise, local changes were lost for good.

Thanks @oguzkocer for pointing out where to make the changes!

[oguzkocer]

Looks good @mzorz! I have left a couple nitpick comments, let me know what you think about them.

P.S: I know I was the one who brought up the changes in handleFetchSinglePostCompleted but I have to say, I am a little bit worried about the potential side effects of it. For one thing, each fetch request will take slightly longer and when a blog post list is visited for the very first time, there will be a lot of these, since we'll fetch each post one by one. Having said that, I can't think of an actual problem with this change, so I am hoping that it's fine :) I just wanted to add my thoughts here in case it proves useful later.

[oguzkocer]

Looks good

I'll merge once Travis is done 🤞