More refactorings. Commented out the plugin tests for now. They seem to wreak havoc with a lot of other tests (especially validator tests) and i havent found the reason yet.

Makes it easier to see the dependencies between these two concerns, cache the results of the query parsing process later on, implement & use alternative hydrators to experiment with different algorithms etc. etc.
Other small refactorings.

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Syntax for getting input properly escaped through prepared statements:
$query->set('field', 'field + ?', $value)
or
$query->set('field', 'field + ? - ?', array($value1, $value2))
or simply
$query->set('field', '?', $value)
Ticket: 405

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