Use fetch_update when generating new IDs

[hiltontj]

Given the limits we impose on database, table, column, etc., this may not be likely, but fetch_add silently wraps around to 0 on overflow. That would be pretty bad if it were to happen. This can be seen in the DbId::new method.

We could instead use fetch_update with a u32::checked_add, e.g.,

NEXT_ID.fetch_update(Ordering::SeqCst, Ordering::SeqCst, |n| n.checked_add(1)).expect("too many IDs!")