Refactor of Zend\Db\Metadata for 3.0.0

[arueckauer]

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?

[weierophinney]

This repository has been closed and moved to laminas/laminas-db; a new issue has been opened at laminas/laminas-db#13.

[weierophinney]

This repository has been moved to laminas/laminas-db. If you feel that this patch is still relevant, please re-open against that repository, and reference this issue. To re-open, we suggest the following workflow:

• Squash all commits in your branch (git rebase -i origin/{branch})
• Make a note of all changed files (`git diff --name-only origin/{branch}...HEAD
• Run the laminas/laminas-migration tool on the code.
• Clone laminas/laminas-db to another directory.
• Copy the files from the second bullet point to the clone of laminas/laminas-db.
• In your clone of laminas/laminas-db, commit the files, push to your fork, and open the new PR.
  We will be providing tooling via laminas/laminas-migration soon to help automate the process.