Avoid redrawing cells when setting separator style.

Fixes #1934 

Setting the `cellSeparatorStyle` causes the table view to redraw its cells. Due to the way the underlying `WPTableViewController` is wired up, this also triggers calls to core data to populate the results controller and a lot of stuff we're not interested in during `viewDidLoad`. 

A work around is to temporarily unset the table view's delegate and data source while assigning the separator style.  This avoids a noticeable lag when switching to the reader tab.