Improve rescinding of offers during quota configuration changes.

Currently, the master attempts to rescind offers when handling an update to the quota configuration. This is problematic because the master's logic is not comprehensive and it's possible for the allocator to race against the master and offer additional resources.

Once offers are managed in the allocator, the rescinding of offers should be done there. The allocator can do a better job at this (e.g. considering reservations) compared to the master's current heuristics.