SNZI counters
SNZI counters

Uses spinlocks instead of crazy lock-free junk because we claim it
doesn't matter.  Implemented between Alewife and Central and tested
between Central and Kendall, so it may be buggy, but it seems to work.

bowlofstew/sv6,aclements/sv6,aclements/sv6,bowlofstew/sv6,bowlofstew/sv6,aclements/sv6,bowlofstew/sv6,aclements/sv6,aclements/sv6,bowlofstew/sv6