Add an AddIfNotPresent function to support single producer/consumer
retry scenarios. Provides the consumer with a means to prefer items
already enqueued by the producer at the point of retry.
Support namespacing in cache.Store by framing the interface functions
around interface{} and providing a key function to each Store implementation.
Implementation of a fix for #2294.
Keep the FIFO's internal set in sync with the queue during Add/Update
operations to prevent a queue line-jumping scenario (described in a
new unit test).
