e3: perf of historical streams

Optimize `StateAsOfIterF`, `StateAsOfIterDB`, `DomainLatestIterFile` method `Next()` to not do `common.Copy`
Write unit-test for next invariant: value returned from `Next()` must be valid until next 2 `Next()` calls. 
In-theory such invariant is enough to survive multiple `stream.Union` `stream.Intersect` compositions