FORMUS3IC_LAS3 / embb

The priority queue relies on setting an invalid time value for
non-existing nodes. However the time comparison checks for valid
embb_time_t types. Therefore prior to doing the time comparison, the
time values have to be checked for validity.

The implementation now uses embb_time_t time stamps instead of uin64_t
time stamps. Thereby time checks now rely on embb internals, instead of
more clumsy comparissons.

Furthermore this allows to use the embb::base::Duration on EMBB level
and thus setting _relative_ deadlines, instead of _absolute_ deadlines
on task creation.

A blocking binary heap based priority queue for tasks with deadlines has been
added. The priority queue sorts incoming tasks according to their
increasing deadline attribute.