Optimize atomic operations in scan kernel template

[adamfidel]

It may be possible to reduce the number of atomics in the scan kernel template algorithm. Currently, we use atomic loads/stores for both the status flags and status values, but it might be possible to only use atomics for the flags and rely on the standard happens-before relations to guarantee that the values would be updated.

Discussed in #1320 (review).

[danhoeflinger]

I agree that the values here shouldn't need to be atomics, and the atomics for the status flags can provide everything we need.