fix(recipe): conn hangs when TreeCache refreshing

The previous implementation of session watcher triggers blocked operations.
If there is a huge tree in ZooKeeper, reconnecting event will lead an
initialized TreeCache into a bad performance state, because the connection
routine was blocked by the session watcher of TreeCache.

This commit put those blocked operations into a background queue to fix this.

There is an example code snippet:

    from kazoo.client import KazooClient
    from kazoo.recipe.cache import TreeCache

    client = KazooClient()
    client.start()

    cache = TreeCache(client, '/a-huge-tree')
    cache.start()

    # Wait the cache be initialized and trigger a connection lost event.

    client.get_children('/')  # The connection is still broken

The patch of this commit has been used in the production environment of
https://github.com/eleme.

kazoo/recipe/cache.py

@@ -189,9 +189,9 @@ def _session_watcher(self, state):
         if state == KazooState.SUSPENDED:
             self._publish_event(TreeEvent.CONNECTION_SUSPENDED)
         elif state == KazooState.CONNECTED:
-            with handle_exception(self._error_listeners):
-                self._root.on_reconnected()
-                self._publish_event(TreeEvent.CONNECTION_RECONNECTED)
+            # The session watcher should not be blocked
+            self._in_background(self._root.on_reconnected)
+            self._publish_event(TreeEvent.CONNECTION_RECONNECTED)
         elif state == KazooState.LOST:
             self._is_initialized = False
             self._publish_event(TreeEvent.CONNECTION_LOST)