refactor and optimize FieldRangeVectorIterator

The implementation can be refactored, and also there has been a note in the code since this class was created suggesting that additional optimizations may be implemented. The optimizations fall into two categories:

• some btree skips that are applied for range bounds may be optimized out for $in bounds (in some cases they are already optimized out)
• in some cases fewer comparisons could be performed by the implementation