model-checker : fix MC_RANDOM simcall

model-checker : fix MC_RANDOM simcall

Time doesn't exist in model-checking. However, there are some
protocols, such as Chord, with some periodics operations (random
lookup, check predecessors, ...).  If the model-checker is used with
these examples, these operations are never executed because of the
time which doesn't progress.  With MC_RANDOM simcall, the
model-checker will explore the two cases : the case in which the
periodic operation is executed and the other case in which it is not
executed.  In this way, the verification is exhaustive whatever the
period.