core: allow spurious wakeups in coio_waitpid

Currently it's possible to wakeup a fiber, which is waiting for a
child process termination, using Tarantool C API. This will leave
a zombie process behind. This patch reworks `coio_waitpid` in such
a way that it yields until `cw.data` is set to NULL in the process
status change callback.

Part of #7166

NO_DOC=refactoring
NO_CHANGELOG=refactoring