Add support for persisting event format versions

[erikjohnston]

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.

[codecov-io]

Codecov Report

Merging #4437 into develop will increase coverage by 0.04%.
The diff coverage is 71.42%.

@@             Coverage Diff             @@
##           develop    #4437      +/-   ##
===========================================
+ Coverage    73.58%   73.63%   +0.04%     
===========================================
  Files          301      302       +1     
  Lines        29822    29824       +2     
  Branches      4894     4896       +2     
===========================================
+ Hits         21946    21962      +16     
+ Misses        6446     6429      -17     
- Partials      1430     1433       +3

[erikjohnston]

This is split out from #4403. I've addressed the review comments that were there, including moving delta file to 53, and adding back in the imports used for auto completion, and generally killed spurious changes.

[richvdh]

lgtm