-Fix for most (all?) concurrency lockups!

-Doesn’t cache tiny images.

-Doesn’t cache navbox images. These were appearing as extra
images in the last TOC cell.

-Retrieves only single core data object when only one is sought.

-Moved image blob data into own ImageData entity so core data can
lazily load it (and not necessarily load it if only accessing Image
object)

-Fix for html clean-ups only happening for first section html
the first time a page is downloaded.

Change-Id: I452ba952bcc0f04ac9920a8c93471e6d12ace421