Allow passing PDO in PersistentObject constructor

Passing a PDO in the constructor means persistent objects are fully
initialized when the constructor returns (as opposed to requiring the
caller to assign the PDO after the persistent object has been
initialized).
Use regular object construction when fetching persistent objects

When fetching persistent objects through PersistentObjectQuery::fetch(),
use the PDO::FETCH_ASSOC style and pass the returned properties array to
the regular PersistentObject constructor (along with the PDO). This
works around PHP's somewhat unconventional object construction approach
when fetching using the PDO::FETCH_CLASS style. (PHP will assign
property values before calling the constructor.)
Merge branch 'feature/persistent-object-queries' into develop
Add @ignore doc comments to operator consts
Merge branch 'feature/persistent-object-queries' into develop