Avoid superfluous ZK exists calls

These cause a major (2x) slowdown in read calls since Zookeeper
connections are expensive/slow. Instead, just try the thing and return
None if there's no key there.

Also wrap the children command in similar error handling since that did
not exist and could likely cause some bugs at some point.

daemon-common/zkhandler.py

@@ -211,10 +211,10 @@ class ZKHandler(object):
         """
         Read data from a key
         """
-        if self.exists(key):
+        try:
             path = self.get_schema_path(key)
             data = self.zk_conn.get(path)[0].decode(self.encoding)
-        else:
+        except NoNodeError:
             data = None
 
         return data
@@ -290,8 +290,13 @@ class ZKHandler(object):
         """
         Lists all children of a key
         """
-        path = self.get_schema_path(key)
-        return self.zk_conn.get_children(path)
+        try:
+            path = self.get_schema_path(key)
+            children = self.zk_conn.get_children(path)
+        except NoNodeError:
+            children = None
+
+        return children
 
     def rename(self, kkpairs):
         """