nova: fix thread in page fault and getting paused

If a thread caused a page fault and later on get be paused, then it left
the recall handler immediately due to the pause call instead of staying
in this handler.

Add some (complicated) state machine to detect and handle the case. Still not
waterproof, especially server threads may never get recalled if they never get
a IPC from the outside.

Fixes #478
run: use gdb_monitor by autopilot

Issue #478