fiber: unref fiber.storage via global Lua state

Fiber.storage is a table, available from anywhere in the fiber. It
is destroyed after fiber function is finished. That provides a
reliable fiber-local storage, similar to thread-local in C/C++.

But there is a problem that the storage may be created via one
struct lua_State, and destroyed via another. Here is an example:

    function test_storage()
        fiber.self().storage.key = 100
    end
    box.schema.func.create('test_storage')
    _ = fiber.create(function()
        box.func.test_storage:call()
    end)

There are 3 struct lua_State:
    tarantool_L - global always alive state;
    L1 - Lua coroutine of the fiber, created by fiber.create();
    L2 - Lua coroutine created by that fiber to execute
         test_storage().

Fiber.storage is created on stack of L2 and referenced by global
LUA_REGISTRYINDEX. Then it is unreferenced from L1 when the fiber
is being destroyed.

That is generally ok as soon as the storage object is always in
LUA_REGISTRYINDEX, which is shared by all Lua states.

But soon during destruction of the fiber.storage there will be
only tarantool_L and the original L2. Original L2 may be already
deleted by the time the storage is being destroyed. So this patch
makes unref of the storage via reliable tarantool_L.

Needed for #4662

(cherry picked from commit 5b3e8a72)