[utils] Fixes compatiblity of QueryCache with expected dict interface
QueryCache is expected to mimick the interface of the dict built-in. The current
__iter__ implementation breaks this expectation by iterating over key, value
pairs instead of only the keys.
This changeset fixes this issue by changing the __iter__ implementation to
iterate over the keys in the cache and providing an implementation of the items
method with a contract identical to the dict build-in (return a copy of the key-
value pairs within the dictionary as a list).