Zend\Db\Metadata\Source\AbstractSource Undefined index notices

(AbstractSource::getConstraintKeys): Add call to
child::loadConstraintReferences() and child::loadConstraintDataKeys()
to avoid triggering PHP notices for constraint_references and
constraint_keys
ZendTest\Db\Metadata\Source\SqliteMetadataTest

New unit test for #3719.
Add Docblocks and annotate unit test as #3719
Merge branch 'hotfix/4241'

Close #4241
Fixes #4144
PR #3719
Merge branch 'develop' of git://github.com/ashawley/zf2 into ashawley-master
Zend\Db\Metadata
Changed new SqliteMetadata test to use memory instead of file
Forward #3719
Merge branch 'ashawley-master' into develop