fiber: review fixes

* use fiber_wakeup() in fiber_yield_timeout()
  to make sure we don't fiber_call() a fiber which
  is perhaps already in cord->ready list.
* don't use diag_create() in fiber_new() if the fiber is
  recycled, this may leak a leftover exception in the
  diagnostics area
* replace rlist_del(&fiber->state) with an assert,
  and fix the places in which a ready fiber can
  be called without first having been removed from the
  ready list.