bugfix: don't prefetch bucket-objects.

original code, when it does namespace.refresh() and updates the pool object
(which is a race-condition) the keyspaceCache refers to Bucket{} object that
still has back-reference to the old pool object. I have removed the race
situations (albeit not completely), and empty out keyspaceCache so that a
new bucket object will be instantiated (using new pool object) when it is
looked up by query engine.