For large instance, the size of a cluster infos event is quiet large
(1MB) and is sent every second. If these events are queued, it can takes
a lot of RAM. The time limit ensure that such events don't stay in queue
if they are not processed quickly, as they are all the same, one in the
queue is enough.

Login happens only with authentication informations, identity name is
now only used for legacy directory name. User name of authentication can
be changed. LDAP users can be imported with LDAP::ldapUid. Users can be
imported in courses, groups... with First name last name.

It has to be sure that all parents are visible and accessible as well.
This wasn't achieved sometimes in the current evaluation logic.
Additionally I simplified the learning path access algorithm thank's
the usage of the stored assessment data.

The NodeEvaluation makes it difficult to use a course node in the
learning path access concept. So it is refactored to a minimum extent,
so that it can covering the previous functionality.
As a replacement a new security callback and a new tree node type is
introduced.