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>

Showing the user a backtrace can be pretty confusing, so just printing
a nice error message makes errors easier to understand for end users.

fixes #121

It was valid for 2 minutes, which can result in the server being
asked too much for something that never changes.

We now ask the trusted server for all keys in 1 request, instead of
asking each server individual for it's own keys.