A fix and a test case for https://github.com/tarantool/tarantool/issues/119

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.
Merge remote-tracking branch 'origin/stable'

Fixes gh-119, gh-120

Conflicts:
	src/lua/init.cc
	test/box/fiber.result
	test/box/fiber.test