MEDIUM: peers: update ->commitupdate out of the lock using a CAS

MEDIUM: peers: update ->commitupdate out of the lock using a CAS

The ->commitupdate index doesn't need to be kept consistent with other
operations, it only needs to be correct and to reflect the last known
value. Right now it's updated under the stick-table lock, which is
expensive and maintains this lock longer than needed. Let's move it
outside of the lock, and update it using a CAS. This patch simply
replaces the assignment with a CAS and makes sure all reads are atomic.
On failed CAS we use a simple cpu_relax(), no need for more as there
should not be that much contention here (updates are not that fast).