[WIP] NNTP Cleanup

Implements a number of changes to the NNTP API outlined in #894

1. [x] Clarify the meaning of "subscribed"
2. [ ] A faster way to build an article list
    - [x] Fetch Indexes
    - [x] XOVER
    - [ ] XOVER Single
3. [x] Message-ID support
4. [ ] Newsgroups formed after a certain date.
use `ErrorServerDate`.