EDF Priority update

The current implementation of priority as an int incur that long runs using EDF scheduling will generate priority inversion, thus a thread that reaches a negative priority will be the highest priority of the system, generating a run-time bug. However, modifying the type of priority to be related to the Life_Span Traits when using EDF (and its derivatives) will generate a lot of overhead when using a 64bits attribute in a 32bits architecture, due to the high frequency of scheduling operations. A new way to reset the priority count should be provided.