Add reviveOffers() and suppressOffers() to the scheduler

[mdevilliers]

// Removes all filters previously set by the framework (via
// launchTasks()). This enables the framework to receive offers from
// those filtered slaves.
virtual Status reviveOffers() = 0;

// Inform Mesos master to stop sending offers to the framework. The
// scheduler should call reviveOffers() to resume getting offers.
virtual Status suppressOffers() = 0;