Conflicts:
	test/box/configuration.result
	test/box/configuration.test

A dead fiber could get reused prematurely or garbage collected twice.

This was because __gc could get invoked way after the fiber has
finished and reused for a different coroutine.

The fix is to always access fibers in Lua by fiber id.

Fix a hang on box.fiber.resume() of an attached fiber which gets
cancelled.
When the child fiber got cancelled before detach, the parent
never was woken up, and would hang forever.
Wake up the parent, and, in the parent, do not try to access
a dead fiber.