Remove device_one_time_keys_count from is_empty() sync checks, fixing sync issue as reported by Nekron

Should make the code a little bit easier to read.

Co-authored-by: Timo Kösters <timo@koesters.xyz>

- 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