Support UndefinedBehaviorSanitizer in CI/test builds

[LucaGuerra]

We have already enabled AddressSanitizer in CI with both unit and e2e tests, successfully as it allowed us to prevent a number of oob reads/writes before they landed. As we increase our testing coverage and clean up the codebase we wish to enable more safeguards and UBSan is the first that came to mind to spot potential segfaults early.

In order to do so, we need to remove some "clever code tricks" that would trip UBSan, mostly regarding misaligned memory accesses. This gives us another opportunity to clean up the codebase. We have already started in #1447 bringing the warnings from 154 to 29 but we need to continue until at least unit tests and e2e give 0 warnings, at which point we can enable UBSan.

[LucaGuerra]

/milestone 0.15.0