Change private methods and properties to protected.

my thinking is that if any other class outside of the `jdbc-metadata-connector` module needs those to be `public`, that should really be another connector & module (independent) altogether.
If, for example, we'd have a new `jdbc-connector`, IMO it's perfectly fine to (re)define those 3 properties & methods and keep things independent. A connector should be an "independent, atomic" unit.

On the other hand, if we just wanted to extend the `jdbc-metadata-connector`, IMO we should place the additional code in the same package and therefore `protected` would be ok.