TLD file urls cached on a per classloader basis. Helps with sharing TLD
files between webapps by placing them in a parent classloader.
Each webapp will be able to retrieve the cached version of the URLs and
therefore only needs to scan its own libraries, the parent libraries will
already have been scanned.
For a tiny bit of performance, we will scan the StandardClassloader at boot
in a separate thread so it should be primed in advance of any deployment.