Fixes for comments for posts loaded via notifications.

Delay's fetching comments until a fetched post is loaded.
Fixes an issue where the post fetched from the objectID could be nil.

Fixes #2345
Fixes #2348 

cc @jleandroperez 
Incoming annoying comment: can we move the `obtainPermanentIDs` call inside the `fetchPost` method? (that way this glitch won't bother anyone using the ReaderService!)

Thanks Eric!