As the DiskSpaceChecker owns a deadlinetimer, which depends on the asio
io_service, we need to ensure it is destructed before the io_service is.
The io_service, and now the diskSpaceChecker as well, are specifically
shut down, in order.
Application is still a singleton that dies after main() ends, but will
have nearly no members at that time.
The "UAHF" one used to be "more equal", mostly just because it happened
to be the first.
This makes them all equally equal.
Specifically this removes the special casing and the enum for the 201708
HF (aka BCH fork-point).
We select the right branch now purely based on the historical check-
points.