Don't remove entries from handle_to_request_map.
Since it's a WeakValueDict, no memory leak will be caused by not
removing entries, and in some cases entries are used even after
COMPLETE messages.

Fixes https://github.com/Yelp/python-gearman/issues/13
Merge pull request #55 from noamraph/master

Don't remove entries from handle_to_request_map.