Limit stack size on Threadpool

[robertdfrench]

The threads that merely copy user requests into and back out of the door don't need much in terms of stack, since they don't recurse.

• lower bound: door_data size limit
• upper bound: default stack size

So if we can find any more accurate measures of upper and lower bound, then we can reduce a lot of memory waste.

[robertdfrench]

Use the builder pattern to set the stack size. I think we can set this pretty low, like in the multiple-KB range. Rust uses some kind of dynamic stack growth thing (having trouble tracking down a source for that)