[python3] use inspect.getfullargspec instead of inspect.getargspec The latter is now deprecated

[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).

[test/statsd] add tests for cubicweb.statsd_logger

[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.

[test/rset] typo

[utils] Fixed issue in clearing QueryCache When trying to add an item in a full QueryCache, the cache tries to make room. It tries to list and remove non-permanent items with a transient counter (number of times it has been requested). However, there is a pathological case where items could be non-permanent but still not have a transient counter because they were added, but never requested. In some cases, the full cache could be flushed, including the permanent items. This changeset attempts to fix this issue by only dropping the non-permanent items that did not hav a transient counter.