Consolidate iterator parsing code

[DomGarguilo]

Fixes #6074

Create new IteratorSettingUtil to consolidate iterator parsing/validation code from table and namespace operations into one place.

This new IteratorSettingUtil consolidates and improves the various checks and validation that were being done in both of these places.

[DomGarguilo]

Any changes made should avoid impacting scan performance. Scans need to efficiently parse iterator config. For other code parsing iterator configuration, efficiency is less of a concern.

This is from #6074. This PR does not alter the iterator parsing for scans.

[keith-turner]

In #6040 there is a new class IteratorProperty.java that parses and checks a single iterator prop key/value. That could eventually be used in these changes.