Add support for persisting event format versions

Currently we only have the one event format version defined, but this
adds the necessary infrastructure to persist and fetch the format
versions alongside the events.

We specify the format version rather than the room version as:

1. We don't necessarily know the room version, existing events may be
   either v1 or v2.
2. We'd need to be careful to prevent/handle correctly if different
   events in the same room reported to be of different versions, which
   sounds annoying.

synapse/events/__init__.py

@@ -18,6 +18,9 @@ from distutils.util import strtobool
 
 import six
 
+from synapse.api.constants import (
+    EventFormatVersions,
+)
 from synapse.util.caches import intern_dict
 from synapse.util.frozenutils import freeze
 
@@ -179,6 +182,8 @@ class EventBase(object):
 
 
 class FrozenEvent(EventBase):
+    format_version = EventFormatVersions.V1  # All events of this type are V1
+
     def __init__(self, event_dict, internal_metadata_dict={}, rejected_reason=None):
         event_dict = dict(event_dict)