Make the initial obstack size very small

[brson]

This looks to be the reason at the moment that we can't scale up past ~10,000 tasks. Right now the size of each shadow stack segment is 500K. We probably need to start this as something very small and double it each segment, like the main stack.

[brson]

With 4K obstacks, I can get 300,000 concurrent tasks with a 32-bit target.

[brson]

b9b9b3e

430,000 tasks now