Refactor of `Zend\Db\ResultSet` for 3.0.0

[weierophinney]

This PR provides changes described in #364

I have two questions from refactoring.

1. Return type hints for implementations of \Iterator::current() is incomplete due to inconsistent usage. Right now, type hints are only documented in method comment. Either leave it or a change is required to match return types.

• Iterator::current() documents a mixed (any) type
• AbstractResultSet::current() documents array|null
• ResultSet::current() documents array|ArrayObject|null
• HydratingResultSet::current() documents null|array|bool

2. ExtractionInterface::extract() expects parameter be of type object. From my understanding HydratingResultSet::toArray() could give provide any of null|array|bool. Not sure if I misunderstand it or if that needs some attention.

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Originally posted by @arueckauer at zendframework/zend-db#370

[weierophinney]

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