Protect against database concurrency bugs

[jimmycuadra]

Something I've been thinking about for a while is that we need to be careful about data integrity with API calls that are susceptible to time of check to time of use errors. There are a few ways to address this depending on the situation. An article appeared on Hacker News today (PostgreSQL anti-patterns: read-modify-write cycles) that covers this problem and the potential solutions (and luckily for us, using PostgreSQL for its examples.) A while back I also opened an issue for transaction isolation support in Diesel that will help us with this. This isn't an issue that can be fixed with a PR, per se, so let's just use this as a meta-issue for tracking TOCTTOU errors we find in Ruma's code, and as a reminder to think about this when implementing database logic.