Issue: Book and Chapter Pages are Slow

Changed this behavior for the next build. It wasn't necessary to add previd and nextid columns, since this information is easy enough to gather from the parentlist > childlist field when there are no more siblings.

These changes, while it was enough to make my head explode figuring out how it would work (much like top_bookid did originally), mean that the majority of the work constructing the TOC is done when adding or updating chapters, and the parentlists and childlists fetched on runtime. On deeply nested books (such as child-books of the VaultWiki Manual), I noted some improvements of ~0.1 seconds. On complex books that contain deep nests (such as the manual itself), if there was any improvement it was likely only due to removing a number of loops that took ~0.02 seconds.

Developers should note that as of 3.0.0 Final, the top_bookid field was dropped in favor of parentlist.