fiber: fix leak on dead joinable fiber search

When fiber is accessed from Lua we create a userdata object and keep the
reference for future accesses. The reference is cleared when fiber is
stopped. But if fiber is joinable is still can be found with
`fiber.find`. In this case we create userdata object again.
Unfortunately as fiber is already stopped we fail to clear the
reference. The trigger memory that clear the reference is also leaked.
As well as fiber storage if it is accessed after fiber is stopped.

Let's add `on_destroy` trigger to fiber and clear the references there.

Note that with current set of LSAN suppressions the trigger memory leak
of the issue is not reported.

Closes #10187

NO_DOC=bugfix

(cherry picked from commit 7db4de75)