Add the ability to get notified of connection loss

When joining a group it is common that the entity
joining that group would like to know if it has somehow
lost that group membership if certain conditions such
as disconnection occur, to accomodate this allow for the
driver to have a list of callbacks which will be automatically
called if the connection is lost.

Change-Id: Iab6acbc6b2934998e51f0618ec66962d77a6ce1c

tooz/drivers/zookeeper.py

@@ -17,6 +17,7 @@
 from kazoo import client
 from kazoo import exceptions
 from kazoo.protocol import paths
+from kazoo.protocol import states
 from zake import fake_client
 
 from tooz import coordination
@@ -25,10 +26,25 @@ _TOOZ_NAMESPACE = "tooz"
 
 
 class BaseZooKeeperDriver(coordination.CoordinationDriver):
+    def __init__(self):
+        self.lost_callbacks = []
+        self._registered_listener = False
+
+     def _on_state_change(self, state):
+        if state == states.KazooState.LOST:
+            # These callbacks should be quick, and not block, so that the
+            # kazoo event loop does not block... If we want this to be able
+            # to handle blocking, we need to use the alternate kazoo thread
+            # and schedule this loop there.
+            for cb in self.lost_callbacks:
+                cb(self)
 
     def start(self, timeout=10):
         try:
             self._coord.start(timeout=timeout)
+            if not self._registered_listener:
+                self._coord.add_listener(self._on_state_change)
+                self._registered_listener = True
         except self._coord.handler.timeout_exception as e:
             raise coordination.ToozConnectionError("operation error: %s" % (e))