Fix #420: fiber.cancel() assertion `!(f->flags & (1 << 2))' failed
Fix #429: Replication relay crash with background=true
Merge remote-tracking branch 'origin/gh-429-replication-crash-background'
Merge remote-tracking branch 'origin/stable'

Conflicts:
	test/box/fiber.result
	test/box/fiber.test
'fio' (cooperative file I/O) package: style fixes

Add comments, renames.
Merge remote-tracking branch 'origin/master' into fiber-file-io