core (memory): Sessions fixes: - avoid memory leak with forgotten _purgeSession in case of conflict: an exception is now raised instead - _purgeSession now cancel the timer if it was not done before - KeyError in profileGet is now explicitly raised throught a failure.Failure - use iterator to iterate session in ProfileSessions

Use this module to temporary store files that need to be integrated to other
projects such as Wokkel.

For example, the changeset that introduced this folder adds RSM (XEP-0059)
support to Wokkel. The files in sat.tmp.wokkel are imported over the initial
wokkel files from sat.plugins.__init__.py