Add Event methods to db api.

Start of adding the ability to store raw events in Ceilometer.
Query Filter mechanism is crude for now. Need to find a more expressive
grammar than this EventFilter thing.

Change-Id: I3c64e3858df756d93dd5bf3c6c9651be25cd2b55
Blueprint: add-event-table

ceilometer/storage/impl_mongodb.py

@@ -551,6 +551,20 @@ class Connection(base.Connection):
         """
         self.db.alarm.remove({'alarm_id': alarm_id})
 
+    def record_events(self, events):
+        """Write the events.
+
+        :param events: a list of model.Event objects.
+        """
+        raise NotImplementedError('Events not implemented.')
+
+    def get_events(self, event_filter):
+        """Return an iterable of model.Event objects.
+
+        :param event_filter: EventFilter instance
+        """
+        raise NotImplementedError('Events not implemented.')
+
 
 def require_map_reduce(conn):
     """Raises SkipTest if the connection is using mim.