[ruby-core:82183] [Ruby trunk Bug#13772] Memory leak recycling stacks for threads in 2.4.1

Issue #13772 has been reported by sam.saffron (Sam Saffron).

----------------------------------------
Bug #13772: Memory leak recycling stacks for threads in 2.4.1
https://bugs.ruby-lang.org/issues/13772

* Author: sam.saffron (Sam Saffron)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 2.4.1
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
----------------------------------------
Per: 

https://github.com/rest-client/rest-client/issues/611

gem install rest-client

```
100000.times.each_slice(32) do |slice|
    slice.map { Thread.new { RestClient.get('echo.jsontest.com/key/value/one/two') } }.each(&:join)
    GC.start(full_mark: true, immediate_sweep: true)
    slots = GC.stat[:heap_live_slots]
    puts "slots #{slots} #{i+=1} threads #{Thread.list.count}"

end
```

Causes unbound memory growth on 2.4.1, this was not the case in earlier versions of Ruby or Ruby master. 

When running through heaptrack and massif-visualizer it looks like all unreclaimed allocations come from thread_recycle_stack

Ruby heaps do not grow during tests, all the growth is around unmanaged memory. 

I feel getting this sorted is urgent, we have seen similar leaks at Discourse. 



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