Minor tweak to servlet config

* Used newest version of servlet 2.5 NS
* Added metadata-complete attribute
Renamed stylesheetCache -> xsltStylesheetCache
Cosmetic spacing fix
QWDepException now extends QWRuntimeException
Change to ThreadLocal usage in IdentityService

Entries are now removed from the ThreadLocal when being
set to null, rather than left as nulls.
Added ThreadLocalCleaner memory leak helper

This bean searches all ThreadLocals and purges any values associated
with the application and its dependencies. This should reduce
memory leakage when the webapp gets restarted.
Added better tracking of OAuth nonces

These are now recorded in the persistence model via a new LtiNonce entity.
A new LtiOAuthValidationService class handles the validation and
timestamp/nonce checking of LTI requests. The ScheduledService
now calls a maintenance method to delete old nonces.