ZF-7953: Zend_Validate_Db_Abstract should use getter for adapter
It would make extensions to the Zend_Validate_Db validators a bit easier if the db adapter was accessible via a getter.
Use case: I need a validator that can validate against the context, so I set it up so that I can use placeholders in the exclude parameter that get swapped out with corresponding values from the context. To get the context values quoted, I need access to the adapter, except that I can't use $this->_adapter, as it hasn't been initialised yet. So I end up having to copy the init code from Zend_Validate_Db_Abstract::_query()