Add explicit ODPOR clean-up phase to DFSExplorer

ODPOR requires that we remove elements from the
wakeup tree and add elements to the sleep sets
at state in the execution only AFTER we've
completed fully the recursive exploration of the
space pointed towards by the wakeup trees; i.e.,
we only remove subtrees and add to the sleep sets
when we're walking back UP the state stack looking
for a point to recontinue the execution.