Don’t pre-calculate row heights

If done synchronously, it can block the main thread for a long time.
And as soon as the view appears, UITableView asks for all the heights, so asynchronous calculation is never done in time to be useful
Store/merge synced reader posts asynchronously
Use the right context to sync reader posts
Merge branch 'reader-performance' into reader

Conflicts:
	WordPress/Classes/ReaderPostsViewController.m
Calculate reader heights without creating cells