More rigorous clearing of image redirect cache

Instead of deleting the cache key, set a temporary
value, so that the key stays "deleted" long enough
so that replag won't cause the key to be repopulated with
an incorrect value from a slave db.

Bug: 52200
Change-Id: I0941c93b76fb0aa8eedb883a3ed1167c6e14d0c0
Merge "More rigorous clearing of image redirect cache"