Add validate() base method

Now that all backends implement a validate that
can be called to check if the backend is still
useable we can add a base method to the base class
that defines exactly what this new method does.

Change-Id: Ica4571970bad2de16ce2071a645abf24c25c110b

taskflow/persistence/backends/base.py

@@ -68,6 +68,14 @@ class Connection(object):
         """Clear all entries from this backend."""
         pass
 
+    @abc.abstractmethod
+    def validate(self):
+        """Validates that a backend is still ok to be used (the semantics
+        of this vary depending on the backend). On failure a backend specific
+        exception is raised that will indicate why the failure occurred.
+        """
+        pass
+
     @abc.abstractmethod
     def update_task_details(self, task_detail):
         """Updates a given task details and returns the updated version.