fiber: make the concurrent fiber_join safer

Prior to this patch a bunch of illegal conditions was possible:
1. The joinability of a fiber could be changed while the fiber is
   being joined by someone. This could lead to double recycling:
   the first one happened on the fiber finish, and the second one
   in the fiber join.
2. The joinability of a dead joinable fiber could be altered, this
   led to inability jo join the dead fiber and free its resources.
3. A running fiber could be joined concurrently by two or more
   fibers, so the fiber could be recycled more than once (once
   per each concurrent join).
4. A dead recycled fiber could be made joinable and joined leading
   to the double recycle.

Fixed these issues by adding a new FIBER_JOIN_BEEN_INVOKED flag: now
the `fiber_set_joinable` and `fiber_join_timeout` functions detect
the double join. Because of the API limitations both of them panic
when an invalid condition is met:
- The `fiber_set_joinable` was not designed to report errors.
- The `fiber_join_timeout` can't raise any error unless a timeout
  is met, because the `fiber_join` users don't expect to receive
  any error from this function at all (except the one generated
  by the joined fiber).

It's still possible that a fiber join is performed on a struct which
has been recycled and, if the new fiber is joinable too, this can't
be detected. The current fiber API does not allow to fix this, so
this is to be the user's responsibility, they should be warned about
the fact the double join to the same fiber is illegal.

Closes #7562

@TarantoolBot document
Title: `fiber_join`, `fiber_join_timeout` and `fiber_set_joinable`
behave differently now.

`fiber_join` and `fiber_join_timeout` now panic in case if double
join of the given fiber is detected.

`fiber_set_joinable` now panics if the given fiber is dead or is
joined already. This prevents some amount of error conditions that
could happen when using the API in an unexpected way, including:
- Making a dead joinable fiber non-joinable could lead to a memory
  leak: one can't join the fiber anymore.
- Making a dead joinable fiber joinable again is a sign of attempt
  to join the fiber later. That means the fiber struct may be joined
  later, when it's been recycled and reused. This could lead to a
  very hard to debug double join.
- Making an alive joined fiber non-joinable would lead to the double
  free: once on the fiber function finish, and secondly in the active
  fiber join finish. Risks of making it joinable are described above.
- Making a dead and recycled fiber joinable allowed to join the fiber
  once again leading to a double free.

Any given by the API `struct fiber` should only be joined once. If a
fiber is joined after the first join on it has finished the behavior
is undefined: it can either be a panic or an incidental join to a
totally foreign fiber.