bug#38136: [PATCH] Make gnus-group-get-new-news a non blocking thread

[dick . r . chiang]

I am very grateful for your interest and testing!

> selecting a deleted buffer

I get this quite a bit.  It occurs when `gnus-summary-buffer` in the main
thread gets usurped by a background thread.  I am either allowing this
important variable to get reassigned before the dynamic-let in 
`gnus-thread-body`
or I am not understanding dynamic-let in the presence of threads.
Incidentally, it's very difficult to point to lines of code I'm talking
about without git{hub,lab}.

> line 1791, because it's expecting "(car gnus-group-list-mode)" to be a
> number, but it hasn't been set yet.

I'll look into this, and add a test for `nntp-open-server`.

I am happy to let this marinate to get people thinking about Gnus's future.  
There are many historical artifacts (like blocking `gnus-group-get-new-news`
and left-field variables like `gnus-secondary-select-methods`) that prevent
Gnus from becoming a viable MUA for more people.

Some other fellow recently posted about an ephemeral group branch which he
somehow got others to test for him in-parallel.  I don't know how he did that
outside the debbugs system.