[utils] Implements __iter__ on QueryCache
In order to be able to iterate over the currently cached queries (so that they
may be inspected and invalidated), this changeset implementes __iter__ on
QueryCache.
[utils] Added new get method to QueryCache
The new get method enables the retrieval an item in the cache by specifying a
default value so that in the absence of an item for the key, the default value
is returned.