Restructure device CKF for readability

[stephenswat]

This commit doesn't change the CKF much functionally, but improves its readability a bit by moving variables closer to where they are used, and by adding smaller scopes. Also increases the clarity of the double buffering solution.

May decrease memory consumption slightly as it encourages the compiler to free memory earlier.

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
40.0% Duplication on New Code

See analysis details on SonarCloud

[stephenswat]

Updated. :+1:

[beomki-yeo]

I think @krasznaa would be OK now, so let me merge it