Stripe the Distribution spinlock

This improves performance in the heavily contended case but worsens
the perfomance in the uncontended case.

Before:

    contended: 2.446s
  uncontended: 0.127s

After:

    contended: 0.234
  uncontended: 0.290

Measured on a quad core Core i7 with HT, using 8 HECs in the contended
case.