Consider pruning online GC review queues for better performance/efficiency
This is a follow-up from !373 (comment 431219483).
As of now, the projected online garbage collector will not perform any pruning on the review queues whenever a resource is tagged (i.e. manifest and its referenced blobs) to avoid being prone to race conditions.
We should consider this as a performance/efficiency optimization if deemed necessary. We should test and monitor the online GC performance in a realistic environment to determine that.