Makes the library computer cache the book list

This is meant to reduce lag since SQL is consistently number one in all the profiles.

This PR is completely untested because I'm too lazy to set up a test DB but it looks mostly fine to me.
The best todo in the code
Op 25-dec.-2014 07:13 schreef "Orange Borg" <notifications@github.com>:

> In code/modules/library/lib_machines.dm
> <https://github.com/tgstation/-tg-station/pull/6555#discussion-diff-22268119>
> :
>
> > @@ -118,6 +157,7 @@ datum/borrowbook // Datum used to keep track of who has borrowed what when and f
> >   */
> >  // TODO: Make this an actual /obj/machinery/computer that can be crafted from circuit boards and such
> >  // It is August 22nd, 2012... This TODO has already been here for months.. I wonder how long it'll last before someone does something about it.
> > +// It's December 25th, 2014, and this is STILL here, and it's STILL relevant. Kill me
>
> No
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/tgstation/-tg-station/pull/6555/files#r22268119>.
>