Introduce acceptOffers scheduler driver API for performing operations on Offers

This is a more general version of the `launchTasks` API, which allows the scheduler to accept an offer and specify a list of operations (Offer.Operation) to perform using the resources in the offer. Currently, the supported operations include LAUNCH (launching tasks), RESERVE (making dynamic reservations), UNRESERVE (releasing dynamic reservations), CREATE (creating persistent volumes) and DESTROY (releasing persistent volumes).