Clean finished threads in Group::Spawner.

[marshall-lee]

This is my attempt to fix #670

[digitalextremist]

I will review this and test it. Thanks for the PR here.

[digitalextremist]

So far I am very happy you patched this, and like what I see. My only concerns are to make sure the test is sufficient to cover all scenarios, which on first glance it seems to be... and wondering if the cleanup at shutdown ought to happen inside the synchronize block or not. I will merge this and make any needed tweaks but otherwise the behavior seems correct.

[marshall-lee]

Some comments on why I had to change shutdown code exactly this way:

• removed Queue usage in favor of simple array because there's no inter-thread communication in which Queue is helpful.
• extracting join call out of synchronize there were two reasons: 1) there's simply no need in this since @mutex is aimed to guard only @group destruction. 2) since i used synchronize inside thread too there's a possible deadlock calling join on this thread inside of another synchronize — and even some tests with sleep were failing until I resolved this deadlock.

[digitalextremist]

I thought you might be avoiding mutex deadlocking, understood.

[digitalextremist]

There is a race condition in the test, on rbx

[marshall-lee]

@digitalextremist I can't reproduce it on my computer but I understand how it's possible. Attempted to fix it in #679
Sorry for inattention, I should had write this spec just like the rest in spec/shared/group_examples.rb.

[digitalextremist]

Your #679 PR is perfect. Thank you