Clear diag, enforce cord->ready fiber policy

* clear diag in fibers, avoid leaks, reused fibers start with clean diag
* the following policy is now enforced: the active fiber must be removed
    from cord->ready, to prevent spurious wakeups due to fiber_wakeup-s in
    the past
* reset fiber->f when fiber is awaiting to be reused in fiber loop,
    instant crash if woken by mistake