Refactor of `Zend\Db\Metadata` for 3.0.0

[weierophinney]

This PR provides changes described in #365

A few questions came up while refactoring. Some feedback is appreciated:

• Setter returns are inconsistent (e.g. ColumnObject and ConstraintKeyObject). Since there are fluent setters, possibility is that they are being used. Therefore I recommend to unify and make all setters fluent. If that is okay, I'll provide the changes in this scope.
• Unsure where class constants of ConstraintKeyObject and AbstractSource are used. Currently the visibility is defined as public. If they are not needed elsewhere, private or protected might be more suitable.
• The $table parameter of loadConstraintData() and loadConstraintReferences() in class Zend\Db\Metadata\Source\AbstractSource is unused. Version 3.0 might be a good time to remove it (causing BC). Should I include this change in this scope?

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Originally posted by @arueckauer at zendframework/zend-db#369

[weierophinney]

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