nuxeo login stack's are not removed from the thread at filter exit

Login contexts are saved in the web authentication cache and referenced in the web session. But, the login stack associated to the thread are not removed once the filter exit. This makes the login stack entries being leaked each time a session is created.

The symptom of this is that external processes that do a lot of automated authenticated queries to Nuxeo will see a slow but steady increase of memory leading to eventual full memory, needing a Nuxeo restart.