Rework DbalReader to reduce memory usage.

Also add several tests about iterator interface

For one of my projects, I needed to pull more than 30k records from db. With previous implementation, it was not working.

refs #66
I don't agree, I use an empty string to only change ```$params``` if needed.
Also, this check has been added with a test in tests/Ddeboer/DataImport/Tests/Reader/DbalReaderTest.php