add worker thread tracking

add stGen which gets bumped for every worker thread spawned by
loadBook

every thread captures stGen when it spawns and before actually loading
the book in the main thread, and in the worker thread it checks if
myGen == stGen

this throws away stale requests, meaning if you try and load another
book while one is still loading, it will ignore the one still loading
and start loading the latest request

app/State.hs

@@ -18,5 +18,6 @@ initialState title path =
       stIndex = ChapterIndex 0,
       stZoom = 1.0,
       stTitle = title,
-      stBookPath = path
+      stBookPath = path,
+      stGen = 0
     }