Fix Mark-As-Read-While-Reading Workflow

Finally putting issue #144 to rest by refactoring the way that all `Reading` activities handle the collection and submission of mark-as-read operations as stories are read.

This resolves all the known issues around marked-as-read status being lost due to context switching.  Tested all the various reading styles (feed, folder, social feed, allstories, allsharedstories) to ensure things now work in real-time.  Did uncover and document one additional existing bug that I couldn't fix at present (see the TODO in FeedUtils).