Shutdown: Do not export routes

Shutdown: Do not export routes

When shutting down, the tables start flushing routes before all protocols
are even officially aware of the shutdown. This fix allows for a fast check
whether the shutdown is already running or not, and if so, all exports are
ignored instead of processing them.

One notable exception is the kernel protocol which needs to process all
the exports normally to actually withdraw the routes.

This is a hotfix for #251 and #252. Proper fix will require protocol
state machine refactoring.