Do not keep cells above view on-screen

For performance reasons, it is not practical to keep cells pinned to the top when they would normally scroll offscreen above.
This change removes the block of code that pins cells to the top of the screen, resulting in a marked performance improvement with cell counts > 100.
refactor how exposed cells are presented

Hide all cells besides the exposed cell, the cell above, and n cells below (n determined by a new property bottomOverlapCount).
This change results in a marked improvement in performance for cell counts > 100.