Miscellaneous pagination fixes

[ricellis]

PR summary

Fixes: s1187

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PR Checklist

Please make sure that your PR fulfills the following requirements:

• The commit message follows the
  Angular Commit Message Guidelines.
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes
• Documentation content changes
• Other (please describe)

What is the current behavior?

The new (unreleased) pagination code delivered in #696 had some bugs.

What is the new behavior?

• Disallow key as an option for pagination
• Fix NPEs for reduced/grouped view results with null IDs
• Fix page size adjustment for key-based (n+1) pagination
• Remove skip option from subsequent pages
• Add new test assertions to check page size against options
• Add new tests for key option validation
• Add new tests for asserting skip option changes
• Refactor the OptionsHandler to accommodate these changes and move some functionality out of the page iterator classes as a result
• Adapt the BasePageIteratorTest to not be an n+1/key pager

Does this PR introduce a breaking change?

• Yes
• No

Other information

[mojito317]

Looks good to me! I left one tiny nit only.