Conduit did not send the event in /sync because of a race condition.
There is a brief moment in time where Conduit accepted the event, but
did not store it yet. So when the client /syncs it updates the since
token without sending the event. I hope I fixed it by significantly
shortening the race-condition period.

json_body is used in places that need authentication. In
case an unknown field is set, Ruma doesn't parse the field
and so doesn't give an error on invalid UTF-8. But Conduit
has parsed and on error makes json_body None. Return an
error to the client instead of generating an internal error.

Fixes #35

- get encryption keys over federation
- optimize signing key storage
- rate limit parsing of bad events
- rate limit signature fetching
- dependency bumps

currently they will only be sent if a PDU has to be sent as well

- power level content override adds to the default event instead of
replacing it
- sending code refactored to make edus possible
- remove presence events when restarting conduit
- remove room_id field from read receipts over /sync
- handle incoming read receipts
- fix array bounds bug in server_server.rs