Backend extensions

[foxcpp]

This is a proof-of-concept to start some work on #322. See last 2 commits. To demonstrate how it works I implemented server support for UIDPLUS extension and also add it to memory backend.

Based on #374. Will need rebasing to remove duplicate commits.

[foxcpp]

Probably it is also a good idea to merge #342 first into v1, then rebase dev onto master and #374 onto dev, ensuring all changes fitting together well and then only this. Gosh, that's a lot of PR dependencies.

[emersion]

Closing because this is superseded by go-imap v2.