Made Stack Size parametrable (for #3760)

[ptitSeb]

Made Stack Size parametrable.

By default, Stacks are 1MB. It is generaly a good default value, but it might be to small for some specific case, where recursion is (ab)used a lot, like for ticket #3760 .
In some other special case, with a lot of thread, smaller value might also be prefered.

[Michael-F-Bryan]

I think it should be fine to make this a global variable here instead of trying to thread the stack size through Tumables.

We should probably use an atomic or Mutex for the variable so we can drop the unsafe, though.

[theduke]

I disagree, we very well may want different stack sizes per instance.

I can think of a concrete use case where users can configure the stack size per workload...

A global atomic won't work when multiple threads are constructing instances.

In general I'm not a fan of a global mutable setting.

This should be threaded through, even though it's annoying.

Note that tunables isn't ideal here either, because that would require constructing a separate engine.

@ptitSeb , does the chosen stack size affect compilation?

[ptitSeb]

I really really really want to stress out that the function call in question is very low level, so accessing high level structure like tunable or whatever other function could be a challenge.

And also, changing Stacksize on the fly should not be attempted. We are talking about stack here.
Maybe I should add some test only allow 1 change of stack size per run?

@theduke no, the stack size doesn't not change the compilation emited.

[theduke]

I'm not talking about changing the stack size for an existing instance, just configuring the size for a newly created one.