Update isWhitelisted to return false if a listing is inside its own init or finalize exit windows

[skmgoldin]

We fire an event for initializing exits, so users can detect this state, but isWhitelisted should detect it as well and return false for listings which have initiated an exit. Otherwise people can initiate an exit, wait until their finalize window begins, then behave badly and defraud people. If you have initialized an exit and your finalize window has not expired yet, you should not be considered as listed.