Use a new memory context when flushing the cache

Use a new memory context when flushing the cache

When the cache's memory context was in over memory state when the
cache was flushed it resulted in LRU cleaning removing newly entered
data in the new cache straight away until the old cache had been
destroyed enough to take it out of over memory state.  When flushing
the cache create a new memory context for the new db to prevent this.