Master/Allocator does not send InverseOffers to resources to be maintained

Offers are currently sent from master/allocator to framework via ResourceOffersMessage's. InverseOffers, which are roughly equivalent to negative Offers, can be sent in the same package.
In src/messages/messages.proto

message ResourceOffersMessage {
  repeated Offer offers = 1;
  repeated string pids = 2;

  // New field with InverseOffers
  repeated InverseOffer inverseOffers = 3;
}

Sent InverseOffers can be tracked in the master's local state:
i.e. In src/master/master.hpp:

struct Slave {
  ... // Existing fields.

  // Active InverseOffers on this slave.
  // Similar pattern to the "offers" field
  hashset<InverseOffer*> inverseOffers;
}

One actor (master or allocator) should populate the new InverseOffers field.

• In master (src/master/master.cpp)
  • Master::offer is where the ResourceOffersMessage and Offer object is constructed.
  • The same method could also check for maintenance and send InverseOffers.
• In the allocator (src/master/allocator/mesos/hierarchical.hpp)
  • HierarchicalAllocatorProcess::allocate is where slave resources are aggregated an sent off to the frameworks.
  • InverseOffers (i.e. negative resources) allocation could be calculated in this method.
  • A change to Master::offer (i.e. the "offerCallback") may be necessary to account for the negative resources.

Possible test(s):

• InverseOfferTest
  • Start master, slave, framework.
  • Accept resource offer, start task.
  • Set maintenance schedule to the future.
  • Check that InverseOffer(s) are sent to the framework.
  • Decline InverseOffer.
  • Check that more InverseOffer(s) are sent.
  • Accept InverseOffer.
  • Check that more InverseOffer(s) are sent.