Use the capability ID to select the stripe

before we used the thread ID, which created more contention.

This improves performance in the heavily contended case. We're
actually seeing a speed-up running on more cores now.

Before:

    contended: 0.276s
  uncontended: 0.289s

After:

    contended: 0.141s
  uncontended: 0.284s

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