`srand` is not accessible in swift. Remove `srand` anyhow as anything
using `rand` for any actual randomness should be considered a bug in
need of fixing rather than viewing `srand` as an acceptable workaround.
This included:
- Removing unavailable inits wholesale if no longer `required`
- Marking a few classes `final` so they could continue using
`Self(...)` rather than `OWSWhatever(...)`
