[pyramid] Drop module-level cache and cleanup looping tasks in tools
And use a LRU cache over cached_build_user function.
This looping task is problematic because it would not be run from within a
WSGI application which does not have a repository with a tasks manager.
This pulls an explicit dependency on 'repoze.lru' but it's not a big deal
since pyramid already depends on this. RPM spec file not update since it does
not even mention pyramid...