Performance: improve performance of oik autoload

[bobbingwide]

A performance problem was detected in oik autoload in the gvg_bulk_update plugin.
bobbingwide/gvg_bulk_update#18

Here's an extract from the issue

The problem that occured was due to the fact that All In One SEO is checking for the existance of the bbPress class for each post that it's handling to create the AIOSEO Overview box in the WordPress dashboard.
gvg_bulk_update implements a class autoload function that attempts to locate and load the required class.
This functionality is provided by a shared library originally developed for oik.

Both All In One SEO and the shared library are to blame for the performance issue. I hope that by improving the autoload logic the elapsed time will decrease enough to allow AIOSEO to populate the Overview metabox successfully.

Any fix applied here should be backported to oik and oik-libs.

Proposed solution
Return early when subsequent calls for the same class are detected.

Note: This will not fix the problem in AIOSEO.
There's also the possibility that other plugins that implement autoload may also be using up valuable resources for each call to class_exists( 'bbPress' )... and all the other classes that are being autoloaded.