core: allow spurious wakeups in cord_cojoin

Currently it's possible to wakeup a fiber, which is waiting for task
completion, using Tarantool C API. This will cause a "wrong fiber woken"
panic. This patch reworks `cord_cojoin` in such a way that it yields
until a completion flag is set.

Part of #7166

NO_DOC=refactoring
NO_CHANGELOG=refactoring