In order to prevent races where two different processes can try to write
the allocations for a consumer, we are adding a 'generation' column to
the consumers table. This will be incremented each time a consumer's
allocations change, and allocation requests whose generation doesn't
match the current value will be rejected.
Blueprint: add-consumer-generation
Change-Id: Ia247f0783c5bbd172aafbfd86372fabeeef557c3
f8bb106674