Iterator parsing code is duplicated

[keith-turner]

The code for parsing iterator configuration from properties is duplicated throughout the codebase. The parsing is not consistent in its error checking.

All of this code could be refactored in the following way.

• Create a single method that parses from properties to List<IteratorSetting> (could potentially use existing code).
• Modify any code that is currently doing parsing to use this new method.

The centralized parsing code could validate the following.

• A valid scope is used.
• The value has only two fields seperated by a comma.

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.

The following comment suggested a possible way to start this.

Originally posted by @keith-turner in #6040 (comment)