BookInfoManager: Minor fixes & tweaks in `collectSubprocesses`
BookInfoManager: Minor fixes & tweaks in `collectSubprocesses`

* Don't run multiple collectSubprocesses tasks in parallel.
  UIManager:scheduleIn doesn't return anything, using that as a boolean trap made no sense.

* Make the in-place removal of collected pids use a slightly more common idiom
  (table.remove in a reverse iteration of said array).

NiLuJe/koreader,Frenzie/koreader,koreader/koreader,NiLuJe/koreader,koreader/koreader,poire-z/koreader,Frenzie/koreader,poire-z/koreader