Changed logic of iterator termination

[ziflex]

Originally, if iterator returned NONE value, it meant, that this is the end of iteration and there are no values.
This approach has one big flaw - it does not work in scenarios, when an underlying source contains NONE values.
This PR changes it. Now, whenever iterator gets exhausted it returns core.NoMoreData error and all consumers should check output on this error.

Before:

for {
   value, key, err := iterator.Next()

   if value == values.None {
       break
   }
}

Now:

for {
   value, key, err := iterator.Next()

   if err != nil {
       if core.IsNoMoreData(err) {
          break
       }

      return err
   }
}

[codecov]

Codecov Report

Merging #330 into master will decrease coverage by 2.1%.
The diff coverage is 55.6%.

@@           Coverage Diff            @@
##           master    #330     +/-   ##
========================================
- Coverage    39.7%   37.5%   -2.1%     
========================================
  Files         216     216             
  Lines        8578    9146    +568     
========================================
+ Hits         3404    3434     +30     
- Misses       4819    5355    +536     
- Partials      355     357      +2