Code review comment for lp://qastaging/~afrantzis/mir/swapper-multiple-compositor-acquire

I guess with all the new swapper functionality we're finding that we need (like, resizing a bundle, switching swapping algorithms mid-flight, multimonitor, and peeking for individual snapshots (maybe even screencasting?)) I'd rather solve the problems in the layer between the BufferStream and the BufferSwappers than by making BufferSwapper have more responsibilities. Hopefully this will keep our BufferSwappers steady state algorithms for swapping buffers, and the 'corner cases' (mentioned above) are taken care of mostly by diverting the buffers once we've ensured sync in the BufferSwappers.