server/librarian_server/__init__.py: make the background-workers system actually work
server/librarian_server/__init__.py: make the background-workers system actually work

This involves making it dependent on Tornado. The key is that we need some way to go
back and see what happened with the worker thread, back on the main thread. There
was no sane way to see if the worker thread function crashed without wrapping
it.

HERA-Team/librarian,HERA-Team/librarian,HERA-Team/librarian