IORuntimeConfig: reject invalid auto-yield and cancelation check thresholds

[durban]

There are 2 changes to IORuntimeConfig in this PR:

• The important one is to reject invalid values for auto-yield and cancelation check thresholds:
  • Negative and zero values are obviously silly.
  • For auto-yield 1 is also incorrect, since that causes a fiber to always yield, and never make progress.
  • (The existing checks are as before.)
• The less important things:
  • I've moved the check into the primary constructor, so that every code path goes through the checks (especially the copy methods).
  • Previously it threw AssertionErrors, but IllegalArgumentException seemed more appropriate.