go-imap-sql v1.0 status

* Current roadmap: https://github.com/foxcpp/go-imap-sql/projects/1
Note: "In progress" often represents state only of my local repository clone, not upstream here.
If you want to help - don't work on tasks in In progress column unless I allow you to do so.

* Plans outline:
- [x] Implement basic functionality of IMAP backend
- [x] Reevaluate standards conformance and fix found issues
- [x] Extend tests to make sure these issues will not reappear in future
- [x] Implement interfaces required by maddy (https://github.com/emersion/maddy/issues/15)
- [ ] Implement support for more extensions (UIDPLUS, QUOTA, perhaps QRESYNC, CONDSTORE)
- [x] Evaluate implementation performance, optimize SQL queries where needed.
- [x] Define compatibility guarantees and design schema upgrade protocol
- [x] Deploy maddy with go-imap-sql to private server for extended testing
- [ ] Fix found problems (#3)
- [ ] Release as v1.0-beta1
- [x] Integrate with maddy in upstream
...

No time estimates since the amount of time I can invest in go-sqlmail varies depending on the moon phase and quantum fluctuations, sorry.

Also, this issue is a good place for questions about status, plans, and other development-related questions. Feel free to ask here if you don't think your question needs a separate issue.

CC @Hexawolf @NamedKitten @emersion