Introduce function/trait method to compute number of messages in the range

[svyatonik]

#2031 (review)

ExactSizeIterator is not implemented for RangeInclusive<u64>, so we can't use .len(). .count() is probably ineffective (but actually maybe rust compiler is smart enough to optimize that?). So we're using construct like this:

					received_range
						.end()
						.checked_sub(*received_range.start())
						.and_then(|x| x.checked_add(1))
						.unwrap_or(<something-else>),

Let's extract all except the unwrap_or() to a separate function/trait method and use it across the code.