[storagefactory] prevent repeating mass queries with getChildPropertyValues()

Improves performance when sending many small files into the device since
GetObjectPropValue request on each new file (i.e. with values not yet
cached) used to trigger query on its whole parent association.
[tests] remember storage root path in StorageFactory_test
[tests] add utility StorageFactory_test::m_queryForObjSize
[tests] add a case testing excessive getChildPropertyValues() call prevention