add a comment about side effects and whether we want them or not

Change-Id: I9cebe8f7979fd0b502fb23e7048b24a5b9ceb5d4
introduce BroadcastBroadcasters() to speedup BroadcastCells()

Iterating over a range and attempting to get a broadcaster for each cell
position is a performance bottle neck. Take advantage of the column's
existing maBroadcasters structure instead.

Change-Id: I5467a64ee3c0b5f430be1f0c4b940d3f71874827