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.
c5a296b10c