Added a TODO in the master for an allocator bug.

Review: https://reviews.apache.org/r/28812
Added missing includes to sorter.hpp.

Review: https://reviews.apache.org/r/28813
Updated the allocator interface to enable unit testing.

Review: https://reviews.apache.org/r/28814
Converted an initial DRF integration test to a unit test.

Review: https://reviews.apache.org/r/28815
Moved DRFAllocatorTest.PerSlaveAllocation to a unit test.

Review: https://reviews.apache.org/r/28816
Moved DRFAllocatorTest.SameShareAllocations to a unit test.

Review: https://reviews.apache.org/r/28817
Moved ReservationAllocatorTest.ReservedResources to a unit test.

Review: https://reviews.apache.org/r/28818
Moved ReservationAllocatorTest.ResourcesReturned to a unit test.

Review: https://reviews.apache.org/r/28819
Simplified AllocatorTest.Whitelist by adding a unit test.

Review: https://reviews.apache.org/r/28820
Documented the expected allocator sorting semantics.

Review: https://reviews.apache.org/r/28821