Preemption

[nicoloboschi]

JIRA: https://dev.majordodo.org/jira/browse/MAJ-4

Reporter: eolivelli @eolivelli

Ths is the ability for the broker to request preemption on a task when tasks for high priority groups are submitted.
The code of the task need to "handle" preemption, and so this feature will be available only for special tasks.
A premptionable task need to check for an "preemption-required flag" and in this case it should request a suspension (see MAJ-3)

The broker needs to tell to the task to be suspended that a request of preemption has come, for this kind of comunication a new kind of netwwork message will be implemented.

It is to be defined the strategy to use to decide which task should be preempted among all the running tasks....