fix computers retrieval with Search:getDatas
fix agent module form
add method to get agents from computer's ids list
fix computers retrieval from static groups
rewrite prepare taskjobs method

* Taskjobstate's actors are prepared from this method instead of each
old specific prepareRun from specific module classes.
* The taskjobs list retrieval condition has been adapted to the new
system of task.
move 'taskscheduler' crontask to the Task class
ensure datetime value are really null in db

We rely on NULL value in DB to check for schedule start and end
activation and the CommonDBTM::updateInDB need to have 'NULL' to be
certain to update a NULL value.
rework jobstate statuses
working on jobs communication

- Prepare every jobstate according to the task configuration.
- Get the list of prepared jobstates for an agent.
- Make sure the prepared jobstates are still valid at the time of
  agent's request and delete those jobstates if not valid.