Make sure that _read_watcher is not empty before start it

Prevent following to happen:
```
  File "cassandra/io/libevreactor.py", line 197, in _loop_will_run
    conn._read_watcher.start()
AttributeError: 'NoneType' object has no attribute 'start'
```

Author: dkropachev

cassandra/io/libevreactor.py

@@ -165,6 +165,10 @@ def connection_created(self, conn):
 
     def connection_destroyed(self, conn):
         with self._conn_set_lock:
+            new_conns = self._new_conns.copy()
+            new_conns.discard(conn)
+            self._new_conns = new_conns
+
             new_live_conns = self._live_conns.copy()
             new_live_conns.discard(conn)
             self._live_conns = new_live_conns
@@ -194,7 +198,8 @@ def _loop_will_run(self, prepare):
                 self._new_conns = set()
 
             for conn in to_start:
-                conn._read_watcher.start()
+                if conn._read_watcher:
+                    conn._read_watcher.start()
 
             changed = True