Fix memory leak when destroying libev connections

The IO objects held a reference to their LibevConnection instances
through the callback.  This reference cycle was preventing the
connection from being garbage collected.

Fixes #93

CHANGELOG.rst

@@ -12,6 +12,8 @@ Bug Fixes
   with shared state when using multiprocessing (PYTHON-60)
 * Add python-six to debian dependencies, move python-blist to
   recommends
+* Fix memory leak when libev connections are created and
+  destroyed (github #93)
 
 2.0.1
 =====

cassandra/io/libevreactor.py

@@ -187,8 +187,12 @@ class LibevLoop(object):
             for conn in to_stop:
                 if conn._write_watcher:
                     conn._write_watcher.stop()
+                    # clear reference cycles from IO callback
+                    del conn._write_watcher
                 if conn._read_watcher:
                     conn._read_watcher.stop()
+                    # clear reference cycles from IO callback
+                    del conn._read_watcher
 
             changed = True