Implement RecordSet index for fast rows access

[VitaliyMF]

In some cases it is useful to have fast access to in-memory rows represented by RecordSet structure (for example, for getting rows by ID values - like Legacy DataRowCollection.Find method).

I think RecordSet should remain as simple as possible (it should not become a monster like DataTable) and it internally should not handle any indexed access. Instead of that, separate class (RecordSetIndex) may incapsulate this functionality; it may accept list of columns for indexed access (may be used not only for primary key):

RecordSet rs;
var rsIdIndex = new RecordSetIndex(rs, "id");
var someRow = rsInIndex[5].FirstOrDefault();

RecordSetIndex should have .Refresh() method to rebuild index in case if RecordSet is changed.