v3: make some types immutable

[oleg-jukovec]

I made mistakes in some types. I used pointers receivers instead of immutable receivers. It leads to inefficient work to GC, see

https://medium.com/a-journey-with-go/go-should-i-use-a-pointer-instead-of-a-copy-of-my-struct-44b43b104963
https://www.sobyte.net/post/2021-10/go-pointer/

But even in spite of this it will be less confusing to make immutable all *Request types, Datetime, Interval, ?